1999 Chevy Tahoe Hesitation and Bucking

I have been having an ongoing issue with the engine bucking and misfiring etc when it rains. Thought I had a bad cap, or wires so I replaced them. Ran fine for a while and now I am having the issue again only this time it's even happening when dry. My check engine light is on but have not checked the codes yet. If I am at idle it's fine and if I am aggressive with the throttle it runs like a champ, but when I am at cruising speed and just keeping the throttle level it begins to stutter and buck etc. In addition when running at highway speeds, 75-80 it can start bogging down and again bucking. Any thoughts would be greatly appreciated. Thanks!