Dogcatcar

I have an 89 GT a/c delitionkit, 76mm MAF, Somekind of uneven headers, 3" Exaust and some other redneck mods. I am having a problem, When iam just crusing in the car hot or cold sometimes it has a studder where the motor is lagging. Like it is back-firing but not pop's,just a no gas feeling.But when i gun the gas and speed up it goes away. I have changed the fuel filter, IAC, TPS, Cleaned my air filterand have ran a fuel cleaner in it, i olny use 93 octane. Could it be the pump going or the valve springs? I just need some idea! Please Help!!!