[Shotimes] ATX Problem

[Cohiba]

SHO Gurus,

I experienced something new in my ATX today... when I take off from a stop,
the car is shuddering pretty bad from about 10-15 mph.  It almost felt the car
just didn't know what gear to stay in. This, of course, got me wondering how
well my rebuilt (three years ago) ATX was holding up.  The problem seems to be
intermittent, so I don't think the issue is actually with the tranny.  Then
something very odd happened; I was cruising on the highway (about 65mph) and
the car suddenly downshifted, damn near putting my chest into the steering
wheel.

I confirmed that this was not a freak occurance because it happened about 3 or
4 more times before I got home, and then again when I just drove the car.  I
also tried it with the overdrive off and the same shuddering happened when
pulling away from a stop, but no problems with downshifting at highway speeds,
actually the car seemed to bog at random times, like it was trying to go into
overdrive.  Any ideas what I can check?  No codes are coming up...   What
sensor/device controls the gear selection for the ATX?  Could this be a VSS
failure?  Any other suggestions?

Thanks in advance for the input.

Cohiba
http://cohiba.users.superford.org/
92 N2O MTX
94 Stock ATX