Weird Tranny/Speedo Problem

94 Probe SE 4cyl with 116k miles. My father in law put a new tranny in this before I bought it from him, and I'm guessing the new tranny has around 10-15k miles on it. I was at the end of a 150 mile trip when the speedometer started fluxuating by 3mph or so and the engine almost sounded like it would stumble or miss ocassionally (~15 sec or so) but it was pretty minor and my wife who was driving didn't even notice it.

Then it started shifting really rough like it was low on tranny fluid, then finally the O/D off light started flashing. At this point I had her pull over and I checked the fluid level. It appeared to be low so I added 1 qt and then limped the final 5 miles or so to the house. Next morning I went out checked test drove it and everything was fine...smooth shifts and none of the other symptoms from the night before. It was overfilled (I must not have seen the correct level in low light), and there were no signs of a fluid leak in my driveway or on the bottom of the car.

So then we head out for another trip and all of the original problems happened again after a 15 mile drive...speedometer, rough shifts and finally the O/D light. Any idea what this could be? I don't know jack about tranny's but I'm guessing something along the lines of a bad sensor, PCM, etc since I wouldn't think the speedo and tranny would normally go hand in hand...???

Thanks for any help, I'm stumped on this one...

Well I found out on another forum that the flashing O/D off light is basically the same thing as a check engine (or trans in this case light). Took it it and got the fault code 452 "Insufficient input from vehicle speed sensor to PCM". That makes sense given the symptoms. Would you just recommend starting with the speed sensor or is there something else that could trigger this type of code?