2004 Mitsubishi Eclipse slow after distributor cap change

I needed a new distributor cap, rotor, and wires. AC Delco installed made my car sluggish and shift wierd, milage dropped. Second was to reinstall the carbon built up parts first removed and it ran fine. The Mitsu parts came in and were installed -strong in 1st and 2nd gear, but sluggish in 3rd and 4th, mpg's fine. Why did it get to 115 in 3rd and up to 135 in 4th before but now 3rd is sluggish and won't go over 95 and it creeps up in 4th? The new parts make everything else great, but this really concerns me. I have done all the required maintanence, tune up about 4 months ago. The car looks and runs new, but is now way below average in performance.
Help please