90 and 91 300 SEs don’t have a stand-alone FPR, it’s incorporated in the MAS control unit between the firewalls near the OVP. Best to find a good used MAS to swap out to see if it fixes the problem before buying, they are pricey.

If they haven’t been replaced in a while I’d start with the cap and rotor. I had a similar problem and that fixed it. A dying fuel pump could also explain your problem, depending on the miles and if the car has the original pump.