1992 Jeep Wrangler it runs ok until you shift into fifth gear then it loosed speed and you have to shift it back to fourth to gain any speed why would it do this?
If it is a 4 cylinder then the answer is that this is a woefully underpowered engine, which has a measly 123 H.P. & 139 Lb/Ft of torque. If you encounter any sort of incline it will loose power very quickly. If on the other hand it is a 4.0 in-line 6, then you may have a partially clogged exhaust system. Take a close look for a bent pipe or have the catalytic converter checked to make sure it is not clogged. It may also be starving for fuel or air. Replace the air filter and fuel filter and see if that helps. It may also need a tune up.