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Automatic transmission revs without shifting--inconsistent

My 94 Lincoln Mark VIII inconsistently will not shift into higher gear. Usually at 45-50 mph but it has also happened at low speeds too. It revs into some high rpms for 10-15 seconds then suddenly shift. Never happened to mechanic and he said diagnostics didn’t show a problem

It has 87k miles, by the way.

Any one particular gear its hanging up in??


No, it’s done at at very low speeds i.e. 10-15 mph but usually at 45-50 mph (2nd gear but usually OD?). It’s been warm or cold.

There are a couple things to check. First has the 1-2 Accumulator and its springs been replaced? The original parts fail in service and adversely affect shifting. If the orignal accumulator has not damaged its bore, it is a relatively simple replacement when you do step 2. Second, replace the fluid with Mercon V. You must also drain the torque converter (there is a drain plug). I have a '94 and doing these two items corrected shifting problems I had.