Hesitation on acceleration after sustained driving

[AeonTorpor]

I'm having an issue here that i'm trying to think through. Here is the scenario

1991 S13 Stock engine stuffs. Walbro fuel pump, nismo fpr, optima battery, new alternator, new spark plugs, wires, fuel filter, air filter, oil and filter.
After sustained driving (30min or so) there seems to be a serious reduction in ability to accelerate.

e.g.; driving at 75mph for about 30 minutes, slow down to 35 to pass through city, upon exiting speed limit is 65mph, can't push pedal past maybe 30% or it sounds like i'm braking. At that point it won't accelerate past ~3200rpm. Turn engine off for maybe ~5 minutes and it can accelerate just fine. (not overheating, no thermostat and huge Al. rad. w/ dual fans) Ambient temp is around 96F.

So I'm thinking the ECU is going into a safe mode of some sort, the thing is that i'm not sure what to start testing. MAF, FPR, Fuel Pump, ECU, O2.

I have a spare ECU lying around to test, but in the off chance that it's not the ecu (i don't think it is), does anyone have a suggestion as the first place to check, or had this issue?

[AeonTorpor]

Anybody?