1996 Buick Century Custom - 73,000 mi - OVERDRIVE/TRANS

Out of the blue, while driving uphill (not very steep) on the freeway at approx. 50 MPH, the engine/transmission disengaged (for lack of a better description)-- but when I put the vehicle in third gear, I was able to drive and proceed off the freeway.

We are not low on transmission fluid. As I continue to drive (from a standstill), engine is able to accelerate and drive fine, but as 40 MPH approaches, the transmission then disengages (closest terminology I can conjure up); but changing immediately to third gear-can continue to drive fine.

Any intelligent response is appreciated. We're not sure if we are having a "transmission" problem; or an "overdrive" problem; or a "fuse" problem; or what have you. Just want to be prepared with information before we take the vehicle to a mechanic so that they can hopefully diagnose and correctly pinpoint the problem.

Thank you in advance for your time and response!