This sounds exactly like what mine did last summer on a drive back from Detroit. I was cruising down the highway and suddenly my speedo (no, not my swimsuit) dropped to zero. It started slowing down because I'd had the cruise control on and that disengaged. I thought "major problem here" for a few seconds until I discovered it still ran okay if I just pressed on the gas pedal. I have an MTX and I see you have an ATX so your trans depends on the speed being sent to the computer to determine the proper gear selection.
Anyway, mine turned out to be a broken VSS (vehicle speed sensor). Since it was a major pain in the rump to change I drove it all summer that way before changing it.
My VSS is all electronic, I think yours has a mechanical speedo cable going to it. With mine, a little PLASTIC (!) shaft that extends into the transaxle broke off. It was broken too far down to extract it with tweezers or anything else I tried. Someone else was able to get theirs out by heating up a piece of metal and "welding" it to the tip of the shaft and pulling it out. I had to take mine to a local shop where the mechanic said he spent 3 1/2 hours taking part of the trans off to extract the broken piece. I was charged $170. The new VSS at least has a brass shaft.
SInce yours is an ATX the location might be different. Mine is on the top of the trans, near the firewall, pointing straight out the top. You can see a tiny part of it if you lay on your back and look up just to the left of the Y-pipe. I used a plumber's basin wrench to remove the main part of the VSS. I have pics of the old and new one not installed if you want me to email them to you.
