I'm working on buying a '90 N/A, but there are a few quarks ..

One of which, I can't figure out ..
After driving around for a while, I will experience a power loss .. anything above 3000-4000 RPM doesn't really give any power at all, and sometimes it won't go above 3000 RPM .. occasionally the check engine light comes back on .. but if I turn the car off and start it again the problem goes away .. even if I turn it off and back on in motion .. 3 seconds of 0 RPM solves it and it won't happen again for hours..

Other then this, the engine runs very smoothly. It has a steady idle, no smoke, ect.

Does anybody have any idea as to what might be causing it?

As far as mods, I don't think there are any. I know it has an after-market CAT, but I don't believe there is much else that isn't stock.

 

sounds like the ECU is in Limp mode, which suggests that there is some malfunction that will report when the ECU is checked.

 

Check for error codes if any.