overdrive kicks out
 

i have an 88 Eldorado, after about 10 miles in overdrive, it kicks into the lower geaer, and i dont get the yellow/orange check engine light, but the onboard computer tells me service engine soon, refer to owners manual. not what the problem is. no codes come up. i changed the transmission filter and fluids, no change. it also runs a bit ruff right when this happens... i am leaning on an O2 sensor, or throttle position sensor, but times are tight with cash. i do a lot of highway driving to work and back. im pretty good at figuring this stuff out on most cars, but this is my 1st caddie, loaded with all the bells and whistles. 4.5L throttle body, distributer, no coil pack. someone said something about an overdrive switch, but i only see it on the console for what gear position i am in PNRD123. I am not all that great when it comes to diganosing trans problems, but it doesnt seem to be mechanical. being from in the mid 80's-mid 50's temp outside, the timeing this happens is right on the $$. if i stop for a bit say 10-15 minutes for gas... it goes back to being fine till about 10 miles again, and kicks back out. i have been keeping an eye on engine temp, voltage out put, and so on, and none of those seem to be a factor.
If someone has any ideas, or answers be great to hear them. guessing it's a sensor.... but out of the million and 1.... if there is a sensor on the trans, i could use the possible location.... or if it is inside?