First check the OD switch/sensor (if you have one)
While a broken speed sensor would certainly cause a return to default mode (including kicking out the overdrive and sending on the light and make it stay inder 3rd gear), you would also notice a problem with your speedo and ABS if you have it (depends on if early or late 95).
But if you don't have these other symptoms, it is more likely that you have a transmission problem. The electronic overdrive (as indicated by the electronic button on the shift handle) may have burned up a solenoid. May want to take to a shop if this is the problem, unless your savvy.
Check the trans fluid level first, if its low, be sure to fill it and/or flush it if dirty. This could help a trans problem such as this. Synthetic trans fluid can loosen up a dirty solenoid. I have seen it before.
Other possibilities might include a bad sensor under the hood (such as TPS) sending a bad signal to the trans. Though this would likely throw a check engine light as well.
You can reset this problem (and any codes diagnostic codes) by removing the negative battery cable for thirty seconds. It will happen again if something is broken, but in the meantime, maybe you can feel the problem before the trans goes into limp mode.
