Laurie --

Your experience with slippage in the highest gear is completely consistent with a worn (or misadjusted) clutch.

One function of a transmission is to lower the output speed in lower gears, but at the same time multiplying torque (to the drive wheels). Thus, less torque needs be produced by the engine or transmitted by the clutch in these lower gears. And when you hit 4th or overdrive, the engine 'n clutch runs slowest, but the clutch is burdened with the highest torque requirement.

The best advice for a "good" job is to resurface the flywheel, replace the throwout and pilot bearings, and replace the engine rear main seal (which, if leaking, could cause a chattering 'n slipping clutch).