Have you replaced the fuel injector seals? Could be a possible cause.
Also the fuel pump control relay. When they go bad, they behave like there's a vapor lock in the fuel system. The relay, under the glove box, white, is meant to pulse the voltage to the fuel pump when the engine is hot to prevent overpressure in the fuel lines. As the engine compensates for the incorrect fuel delivery, the engine will surge and fluctuate idle, sometimes stalling, with re-start a problem. In hot weather it should have a click-click sound about every few seconds. When bad, they don't click at all, the contacts seize, giving continuous voltage to the main pump.
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'89 245 Sportwagon, VX cam,K&N, Bil's, IPD susp & exhaust