This morning when I backed out of my driveway I put my car in drive, a second later there was a clunk and at the same time my STC icon came on and the STC Service Required message came up. I hit the gas, but the car would not move, it just revved. I tried again a second later and it moved forward, but heavily and sluggishly as if it didn't want to go. I pulled off the street and turned the car off. When I started it a few minutes later all urgent icons and messages were gone and the car moved forward normally. About 4 hours later I was driving and as I slowed to stop at a red light there were two clunk, clunks and then a split second later the STC icon was on again, the STC Service Required Message was up again, the RPM reading dropped like a bomb to below zero and this is really weird, but the A/C was on and instead of blowing out cool air as it had been it immediately started blowing warm air out of the vents. When I hit the gas the car would not go forward. It was as if the car was still running, but had no power or it was in neutral. I had to put the car in park, turn it off then turn it back on to get it moving again. About a 1/2 mile down the road I heard the clunk clunks again as if the transmission was slipping. Currently it's sitting at the Volvo dealership waiting for Mon. morning and hopefully a diagnosis because currrently it feels unsafe to drive. Am a little concerned because in the past year I've spent $3300 on various sensor replacement, scheduled maintenance, and things that are just going wrong with a vehicle that has 100,000 miles on it. Is this typical????

Hi ,similar symptom that I had early last year ...few days after carried out schedule maintenace including replacement of transmission fluid. Defect was intermitted and reproduced almost everytime at green traffic light!!! so I pulled the car to one of a dealer enroute. Fault diagnosis was shown one of a sensor (sorry I can't remember exactly but it was a one that mount aft of the air filter box with air sensing lines) ,however the fault was cleared after they remade its elect conx. Defect gone away for about a week time but reappeared again while I backed out from the garage , then I checked that elect conx myself and found it was slightly loose due to its lock spring wasn't engaged fully to locking groove.
So after 2nd remade that conx, the car is very fine since then..besides I'm keep checking every elect conx inside the engine compt for security as much as I can reach especially after major schedule maintenance...to make sure that all are fully in and can't pull out easily.
I'm wishing that your case wasn't due to tranmission gear itself.
Good Luck
:amen: