Re: 88 2.9 runs rich and loses power.
"Ever since" leads me to believe that you might want to check over the cap/rotor you installed (what was involved in your tune up?), check your firing order, and make sure your o2 sensor wiring is not melting up against your exhaust. Just generally look over where you were, if it ran worse after a tune-up, something either isn't quite right or something was distrupted. What brand of compnents did you use?
1987 Ford Ranger Short Bed 5.0L/T-5 Project
1988 Ford Ranger Long Bed Parts Truck
1993 Ford Tempo 2.3L OHV HSC Auto - D/D Stock.
1999 Honda Accord VTEC Auto
2002 Dodge Caravan - 3.3L Auto