Help with Isuzu Rodeo 1995 V6?

Ive been having problems with my Rodeo for months now. About 4 months ago, about once a week it would bog but be ok after it sat a little while then it would be ok for a few weeks. Then it started acting up in the past 3 weeks on long trips. Now for about a week it is getting worse. When you go up a hill or at a light when you take off it dont want to go, the engine stays running but you can have the pedal to the floor and it just chugs really slow until you get up to about 30mph then it works fine with very little bog and the check engine light has never came on. I have replaced, spark plug wires, fuel filter, cleaned throttle body and replaced egr valve. Also had fuel pump checked, coil packs, and spark plugs all checked fine. My exhaust was pertty rusted and muffler had a big hole in it so we cut if all off after we installed the new parts because alot of pepole was saying it was the converter so we cut that off too only because we need a whole new exhaust. The converter is good, so we took it to a mechanic and he checked over everything. They couldnt hook it up to a computer because the last owner took the thing you plug the computer in out. But the mechanic told me that the exhaust can meke it run like that. The muffler has been rusting away for awhile. Any one know if the exhuast can cause this problem. Ive all ready put alot of money in this SUV and im tired of people just guessing and im about broke to put more parts in it and thats not the problem. I having a new exhaus put on tomorrow any ways but Im not sure if thats causing the problem Thanks