I Have just bought myself a 1995 2.5 960 and i have a problem with revving over 5000 rpm, if i keep my foot about 3/4 of the way down the car revs ok out to the red line,but if i use the full throttle range it struggles to rev above 5000 rpm, i checked and it has a new air filter, i will check the plugs tomorrow, i had to clean the idle valve as it was gummed up.
Now, is the timing set through the ECU or have you to set it manually, could it be the air mass/ throttle body needing cleaned out or could it be the lambda sensor, i am sorry if this seems like a lot of questions but it is bugging me as the rest of the car is superb, Ta in advance. :)

Is the check engine light on?

It could be many things.

My first look would be at the throttle position sensor.

No check light, and i have checked the plugs and they are new as well, i will take the air mass off and give it a clean and look at the throttle position sensor, its a pity Haynes don't do a manual for the 960 as i am shooting in the dark with any procedures and settings.

5k rpm on a 10-11 year old car.. doesnt sound to nice.
edit: didnt mean to quote

Well i have cleaned the air mass sensor and checked the plugs (new), and it is better than it was, i will have a look at the throttle body tomorrow and see how it is, i still think it might be the lambda sensor, but it looks a pig to change, no matter how old the car is it should rev out to it's max power, i don't think i could use a car i could not trust in an overtaking manoeuvre, it might get worse and happen at 4,000 rpm in the middle of an overtaking manoeuvre and leave me on the wrong side of the road.

Blocked exhaust or CAT?

I had a similar problem with my 94 940 and it was intermitent fuel pump failure of the in line pump (most Volvo's have one in tank and in line). My vehicle would run fine untill warm then begin to stumble and have trouble reving. I crawed under, did not hear the in line pump running, checked the voltage to the pump, and after I bumped into the pump with a wrench it began to work. I drove the car around and every time it began to act up I just whacked the pump. I replaced the pump and never had anymore trouble. The cars will usually run off just one pump but will not have power and will not rev. It took me a good while to track the problem down seeing that the previous owner had taken it to several mechanics who could not figure it out, and had even changed the in tank pump but ignored the one in line so I did not think to look at teh pump at first and instead wasted a lot of time running other tests.