Looking for some advice. Car is '98 coupe A4. Had the transmission rebuilt 10-15k miles back. Now lately I have had this happen twice in about 5 months. During highway driving, 70-75mph, the transmission will what seems like a down shift and increase engine rpm by approx. 300rpm. Seems like it happens around the same time as going over some bumps. If you let off the gas and ease back onto it, there is no change. However if you just try to accelerate, without letting off prior, it will so to speak upshift and drop back to regular engine speed. Checked transmission fluid, fluid level is fine, clean and not burnt smelling. What else could it be? Converter? Electrical/Ground? (all is stock other than some upgraded parts in transmission). Only trouble code I have and get is for a left rear wheel speed sensor and I believe it is the wiring at the connection to the hub.

Thanks for any insight you can provide.

 

Sounds like your converter is unlocking/locking up ---
This happens sometimes when driving on a rough road-The usual suspect it the brake pedal--IF the brake light switch is activated it will UNLOCK the converter --It will also DIS engage the cruise control as well- Sometimes the brake pedal will bounce on a rough road--or while driving over RR tracks----believe it or not--I have seen something as simple as aftermarket floor mats cause the brake light switch and pedal to engage !!-
2ndly --ANY misfire whether false or real will UNLOCK the converter-- Check for misfire codes----plugs and coil wires--False codes must be "tunes out to avoid repeating

Also the trans has to be up to operating temp before the TCC will lock up- when not up to temp the TCC will sometimes work mysteriously--( sorta like a blonde chick)---LOL
I wouldn't worry about it--unless it starts to happen all the time--then i would remove the pan to see if there is any metal/clutch material in there--
PS: a tune would help the trans shift crisper-- make the TCC lock up harder- and lengthen it's life