engine will not rev up
 

My 1990 non turbo sometimes will only rev to about 3000 rpm and then start misssing has also done it while driving but by shutting the engine off and restarting sometimes goes away

It sounds like the OMP is going( Oil metering Pump).It will put the engine into limp mode and you get a real big lack of power so you know that something needs to be attended to.
You can pull codes on your engine as it is an S5.
Try this:
RX7 FD and Series 5 Error Codes

having the same issue sounds like the omp but also check the ecu because a bad omp could damage the ecu as well:icon_tdow