1995 Chevy Astro Atro Van Problems

We have a 1995 Astro Van that we bought from a friend. They had just replaced the motor with a GM factory replacement. When we got it, the idle was a little rough and sometimes when you stepped on teh gas, there was a hesitation like it was bogged down, sometimes it would clear itself and then run along fine. Last week, the hesitation got worse. The van would jerk and backfire, then quit running. We replaced the fuel pump and filter and nothing helped. Next we replaced th coil, still no better. The gas guage also reads near empty, but the mechanic said there was about 100 pounds of gas in the tank. It starts when cold just fine, idles ok, then you drive it about 1-2 miles and it jerks and shutters lviolently, backfires, and just dies. Will not start back up until it's cold again. Then the same thing happens. The mechanic says it's losing a spark somewhere when looking at the coil (?) And thinks it needs an ECM. Is this it or is it something else? Don't want to keep throwing money at it.