1996 Chevy Lumina My engine is almost dieing and then surgi

My van has 198000 but the engine we replaced in it only has 60000. So anyhow when I am excellerating my van starts to grab and won't excellerate and then it catches and goes and then it grabs and then it reexcellertes. I was wondering if you could identify the problem. I have researched it on line. I have taken it to a mechanic 3 times and he has "fixed" the problem 3 times to the total of 1200 dollars. But the problem has never been fixed. My husband works out of town and he knows he has me. Can you tell me what the problem is so that I can go in there and tell him what to fix.

Thank you