So it is fine in the morning? So the problem only happens once the motor is warmed up and in the afternoon?
Check the Coolant tempt switch. That might be tweaking out.
Also, it might be fuel pump related. Have you changed out the stock one for a Walboro yet? No, it won't flood out your engine or cause rich problems or whatever. There are way too many people, including myself, driving with them everyday with no problems to worry about it.
Anyway, for some reason, dieing fuel pumps like to work better in the evening and morning when it isn't so hot. It might just be difference in the way people drive during those hours, I don't know. But it is something to consider.
Also, the power band isn't as low as you might be thinking it is. If you've put in a new clutch recently, then you are dumping off of it too hard. The old clutch would have slipped some and compensated for that, but a new one will just grab and hold on.
Beyond that, we're gonna need a little bit more detailed history.