Stuck transmission solenoid code

Has a stuck transmission solenoid code. However, it does shift okay, but if you put try to accelerate with any more than a granny foot the engine light will begin to flash and there's almost no power whatsoever. Like driving a bug with only one cylinder. If you continue the whole car will just shut down. After this happens, we also get an odd harmonic humming in the intake box at idle. The guy has been driving it for a year like this just granny footing it. It'll get up to highway speeds just if you do it very slowly. When it started his oil pressure gauge stopped working, however it can't be that. There's no ticking or rattling of lifters and it revs fine in park or neutral. The whole thing is a mystery. All gears are working. The humming that comes and goes from the air breather before the filter is even weirder. Never seen anything like it. No loss of oil or coolant, transmission fluid smells off but is full. Idles well. The only thing is a stuck solenoid code and a bad oil pressure sensor which is common. It kicks the gears down like it should but all power in the engine is way reduced. What could it possibly be?