No Power Sometimes

My 90' I6 180K has nothing from 20-45mph,when you give it gas, its hard to tell that your pushing (GRAD'L) down on the pedal. It started doing this a while back,then all of sudden it runs good,WOW! & then back, there is no miss,or popping or farting,just little p/u & go. I've only had the jeep about 7mo's & have put,plugs,crank sensor,thermostat in. Its nice when runs right!!! THANKS IN ADVANCE!

I'd suspect cap and rotor and plug wires.