Will an ECU fail intermittently?

My car is a 91 740 turbo 5 speed and it will occasionally not rev well over 3000 rpms, I have changed the Radio interference relay, FPR, coil, plugs, wires, cap rotor, AMM, Fuel Filter, turbo hoses and had a mechanic verify (with a pressure gauge) that the in-tank pump is working properly. since doing the FPR and the turbo hoses the problem has happened with a lot less frequency.

My only other theory is that the center muffler which is shot might have had something come lose in it that was creating a partial blockage.

the mechanic said the cat seems fine, he thinks maybe there is something up with the ECU but said they generally will fail outright.

The check engine light does not come on, nor does it throw any codes when this happend.

Any thoughts would be apprecaited.

The car will be with the mechanic for a few days, so I can not see what the number on the ECU is.