I was given this volvo after it sat for 3yrs. I replaced the fuel tank, both pumps, filter, complete tune up, cleaned PCV system, Cleaned fused box, and new air filter. One constant problem it has is, about 50% of the times I drive it, it will suddenly lose power. It doesn't stall, just looses power. When I press on the gas pedal it just boggs down. When I shut the engine off and let it set a few seconds and then restart it, it runs fine with full power. Some days it not a problem and then today it must have happened 7 times while driving a total of 50 miles.
Thanks for any suggestions.

When you did the tune-up, did you change the ignition wires, plugs, rotor, cap, or what?

new plugs, wires, cap and rotor. I also want to add that 3 different garages who were suppose to be volvo experts have not solved this problem. I don't know if this is a very complicated issue or it is so simple that it is being over looked.

May have an obstruction in the exhaust, like a bad cat. Happened to me. Loss of power turned out to be a bad cat.

Or the air mass meter. Unplug it with the power off and then take a test drive. It will run rough but if it's the problem then you shouldn't have sudden loss of power (with it unplugged).

I checked for obstructions in the exhaust system and it appears to be clear.
Where is the air mass meter located? Also, 90% of the time this car runs great and every time it messes up, turning the switch off for a few seconds resets whatever the problem is and it runs great again. One other thing I forgot to mention is that when I fill the gas tank up the fuel gage registers below empty and it will run really bad for about 50 miles or so then the gage registers full again and it runs good again.

Check fuel pressure-your fuel pump relay "may" be faulty, ditto your fuel pressure regulator-if it's bad it can't run well. AAM is midway between butterfly housing and air filter, beside the Left F suspension tower.

I replaced fuel pump relay and problem not solved. Checked fuel pressure and its good.

It sounds like the same thing thats happening to me. Any improvements?

When was the fuel pressure good ? when the car is running good or bad ? it sounds like the intank fuel pump could be the problem I mean not working when the fuel gauge reads below empty.

When the machanic installed the new gas tank and intank fuel pump everything looked good to him. I have tried most of the suggestions so far and it still looses power but its not as frequent. Sometimes I drive it for two or three days and no problem, then it happens.
Something that I forgot to mention before is when I turn the motor off and restart it, sometimes the overdrive light comes on for a about 15 seconds and then shuts off and everything is OK. Don't know if that has anything to do with loosing power.

Thanks for the continued suggestions

If my car was doing that I would go to the pick-n-pull and get another air mass meter and swap it out. If the problem persisted I would go back and grab another fuel ECU and try it.

This morning I drove my car through a dip in the road. It temporarily stalled and then fired up and kept going with full power. The transmission was in drive and I assume that is why it started back up. I drove it home (5 miles or so) and no problem. I shut it off and 1 hour later tried to start it and it would turn over OK, but would not fire. I tried numerous times and nothing.
I think it is time to call the junk man.

clean ALL the fuses and holders-esp the sealed one by the left suspension tower. Aluminium fuses are the bane of old cars, they corrode and obstruct conductivity. B$ cleaning them check voltages at fuel pump and fuel relay, verify no voltage drop across fuses 4 and 6 with ign on.. festoon fuses are crap, suggest replace all with blade type holders and fuses. report pls.

The fuse holder on inner fender was broken in half. I cleaned it up, used some duct tape and it started right up. 1 problem solved and one to go. Thanks for the help.
Later I drove the car about 10 miles and the same problem with loosing power happened again. I put it in neutral, turn the key off for about 5 seconds and started it up again and it has full power, (until the next time).

What you're describing is classic fuel pump overheating. I'd go back to the mechanic who installed the pumps and ask him to recheck all his work.

Hey, if you are cranking it over and over again and it won't start. Pull off the right side panel(on the passenger side) by your feet so you can see the ECU, now before or during give the ecu a wack and if it turns on, well your ecu is going out and you found your problem I would do it while cranking so you can make sure that is what it is because once you hit it, it will start. let me know if that is it. hope this helps.

Oh yeah what is your mileage on the car?

I don't have a problem starting the car. After it is running for awhile it will suddenly lose power. It doesn't stall out just looses power.
I bought new copper fuses for the intank pump as well as the other pump. Tomorrow morning I leave for a 2,000 mile trip with this car, so I hope it is something minor. I put another can of STP miosture remover in the tank, changed the oil and filter and I got my fingers crossed. Hopefully that will work.
I'll report back after Nov 3rd (From home hopefully).