1999 Lincoln Continental transmission sleps

Occasionally the transmission slips sometimes a little or a lot like up two to seconds; only notice these happing when taking off from a dead stop. The car is at operating temperature when this happens. Took the car in to the ford garage to address drivability problems about a year or more ago such as fore high idling which they fixed somewhat while I was there I told them the Tran slips they checked for codes found none and test drove it and of course it did not do it when tic test drove it. I’ve changed filter and fluid before taking to garage it was doing these somewhat before I changed fluid. The pan looked fairly clean when pan was removed replaced filter filled the Tran with mobile1 Tran fluid flushed Tran out through cooler lines and shifted car through the gears wile holding the brake pedaled down added over twice the normal amount of lube guard and one tube of DR. Tranney to it as well. Also added lube guard Tran flush with some fresh fluid first before and ran engine and went through the gears before taking the pan off and flushing the old fluid out as much as I could: Without over filling Tran. The last time Tran fluid was changed prior it had I think 37 0r 38 thousand miles on it. The car had about 107 thousand miles on it when I changed the fluid. The old fluid did look brown but did not smell burnt. Any thoughts what the problem may be? Thank you for your time