1996 Hyundai Elantra warm weather miss

Vehicle runs smoothly all winter. In warm weather engine at running temperature at highway speed begins to miss badly. Will then only run at 25 mph missing badly. The feeling is same as if it running out of gas, or vapor lock on carburated vehicle. Engine is fuel injected. Overheating is not the problem. Tried lowest temp thermostat available and then removed thermostat entirely. Engine temp reaches halfway mark with no thermostst but still stalls and misses. No local mechanic has found problem. Most attempt were to clean fuel system, injectors. Fuel pump replaced.