Engine rpm fluctuates at constant throttle

I have a 1998 gmc sonoma with a 2.2 litre 4 cylinder engine. I recently had issues with engine hesitating at stop signs so i replaced fuel filter as well as fuel pump like the mechanics had told me that was the issue. well it wasnt the issue. i later found out that i had a cracked spark plug. so i replaced plugs and wires and problem went away but i have noticed the engine rpm fluctuates at constant throttle. engine is throwing no codes and have replaced o2 sensors, map sensor, air temp sensor, throttle body cleanup has been performed, and tps sensor within the past year. i have also checked fuel line pressure and it reads within recommended specs. transmission has also been rebuilt a couple years ago. please help