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Hi Mark, My name is***** will be glad to try to help. Check the oil condition for burnt smell or dark discoloration indicating an internal failure and excessive heat. Also drop the pan and inspect for sludge, metal, debris indicating an internal failure occurring requiring overhaul. If clean I recommend having the transmission pressure checked to see if it drops off in 4th gear indicating leakage in that clutch pack. Let me know
sounds like either there is a problem in the valve body for the 4th clutch engagement or the 4th gear clutch. When things are at temp they expand and the oil viscosity thins out. Having the pressures tested will allow any pressure drop off in 4th to be seen.
See if the pressure drops off when this occurs.