NO POWER 20 TO 50 MPH

I've only had my 90',I6,XJ for 7mos,put plugs,crank sensor & thermostat in,now sometimes when you give it gas. its very sluggish,like when you pull out on a road step on it,its hard to tell when you arre pushing it at 1/3 throttle or 3/4's,& then at times it runs great! that's nice! but thats getting fewer & fewer! forgot 180k,THANKS IN ADVANCE!!!!!

sounds like a fel pump to me, but im not an expert

New filter, clean the throttle body, add a can of BG 44K to the tank, correct plugs and make sure there is no vacuum leaks (especially in the MAP sensor line) If the TPS is going bad it can act that way too.