My st after about 20 min won't rev over 4000' I've changed air flow, both lamba sensers, temperature sencer, new plugs, once all codes are cleared it runs fine for 20 min or so then bk to running crap, the car seems to run better as I ease off the accelerator, this happens in each gear.foot to the floor runs shuddy as I ease off runs fine, but you can tell the lack of power is missing, I've had numerous of my friends look at it who no a bit about cars. Any more ideas plz what could be problem.

Many thanks.

The ST170 has variable inlet tracts/ runners(aka variable velocity stacks or belmouths) these lengethen for low revs and shorten for higher revs (different, but has a simmilar effect to variable valve timing) it they are inoperative or jammed at the "low" position the engine won't want to rev at the top end

There may be other reasons the why engine won't rev (eg - restricted breathing like a blocked or dirty air filter, or the exhaust partially blocked) but the variable inlet is a common problem on these cars and worth checking

The car is all stock and you have not fitted a cone filter/ non standard exhaust or anything, right?

Could be partially blocked injectors. Had the car been sitting unused for a while? The injectors start to tarnish. When Ive had this happen before I found driving around in the low rev range after putting some redex in the system eventually cleared it.

What codes are you getting?