I may be wrong but i think the whole point of the cooling systems upgrade for 92-01 is that it doesnt get air pockets only 87-91 got the air pockets. I have an 89 and its did the same thing and then i upgraded my cooling system to the one they have on the 4.0 high output and now i have NO problems what so ever but you already have that cooling system so my best guess is completley flush your system. Empty every last drop of coolant and use a radiator flush or radiator cleaner and follow the directions. After you do that fill it with a good quality 50/50 mix coolant and then add royale purples Purple Ice. Purple Ice is a high performance, synthetic, radiator coolant additive. It reduces engine heat by reducing the surface tension of the radiator fluids for improved heat transfer. It also helps to prevent the formation of scale deposits in the radiator for better coolant flow, and it lubricates the water pump seals.