Loosing power at 2.5k rpm

Having a bit of an issue with my r32 gtst, intermittant issue that doesnt happen everytime i drive however it is getting more and more frequent and generally when the car is HOT, it will top out at 2.5k rpm and shake and jump and generally have a fit, if i gear down which would normally put it up to 3k+ rpm it will just do nothing at all untill it comes down to 2.5k rm then top out again. Very annoying and embarrassing, very rarely it will top out at 1k rpm and i pretty much cant drive for 10 minutes untill it is happy again.
I have replaced my fuel pump and checked the plugs and leads, other than that i am guessing maybe an air/fuel ratio issue.
I did replace an alternator a while ago which incurred a few jump starts and a new battery, maybe i have put it out of tune??
Exhaust definately smells like it is running rich and gives off smoke, grey i think.
Other than that i have got no idea at all, any suggestions would be great, no major mods just air filter and exhaust.