well i was on the interstate last night an i noticed wen the cruise was set or wen u keep it a steady speed it felt like it was like chugging but the rpms didn't jump around as if it was misfireing an theres no check engine light what could be wrong

 

A misfire would not cause the RPMs to jump around unless it was pretty severe and affecting multiple cylinders.

When was the last time a tune up was done (plugs, wires, cap, & rotor)?

 

it was about 6 months ago or so