1990 Mazda Miata won't go, but will overheat

I recently replaced the plugs, wires, air filter and pcv valve on my 1990 Mazda Miata with 100k miles because it was running a little rough. It helped very little, so I also replaced the fuel filter, which helped a little for a few days and then I lost almost all power and acceleration. If I put my foot to the floor, with the clutch engaged, it can take up to 10 seconds for the rpm's to get up enough so that I can go. Then I may only go a few miles an hour until it gradually lurches its way up to speed. After driving no more than 15 minutes, it will overheat.
Spark, compression, converter and fuel pressure at the filter all check out fine. The shop it's at now wants to try the fuel pump, but isn't sure that will do the trick. They say the pressure is at the low end of normal.
Does it sound like a fuel pump? COuld it be the fuel pressure regulator? Fuel injectors?
Help! They don't know what to do next.