engine loses power
 

1990 non turbo will lose power and not turn over 2500 rpm if engine is switched off and restarted will come back to full power only to cut out again seems to happen most when car has been driven a short distance and warmed up and then shut off and restarted

sounds like the Electric OIL METERING PUMP is going.
you can pull codes on that series of car.
here:RX7 FD and Series 5 Error Codes

Definetly omp. Is your check engine light coming on when this happens?

the check engine light has not come on is the omp difficult to replace?

Not really. 3 bolts and two plugs. Just make sure not to pinch the oring