I have a 1995 Isuzu with a Chevy motor when I start the engine the car runs fine

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When the engine gets warm. The biog so we'll start running really really bad then it in reverse it runs bad almost all the time I've replaced a lot of parts including the fuel pump and clean the accelerator table and I'm still having the problems I had one mechanic tell me that it was the timing chain but it doesn't do this all the time not until it warms up before this happened it was running fine I had no problems now it does it all the time and I tried everything and taking it to three or four different mechanics and they all say something different