94 STS randomly changing gears

94 STS with 4.6 liter V6. OK when cool. After warming up it randomly jumps from one gear to another when above 30 or 35 mph. It's not the converter locking and unlocking because it's jumping down to second and third also. It's not showing any trouble codes. Would appreciate any advice.

You need a full trans scan with a scanner set up to access the trans functions, this will point to any solenoid or slippage issues, if none a regalar scan could point to an issue with the tps.

You don't need a scanner to run diagnostics on a '94 STS because OBD I is built in. Do the following to check for trouble codes:

To enter diagnostic mode:

1. Turn ignition "ON." (Don't have to start, just turn ignition to "ON" position)
2. Depress the "OFF" and "WARMER" buttons on the Climate Control Center (CCC) simultaneously and hold until the segment check appears on the Instrument Panel Cluster (IPC).

Here is a link to the Trouble Codes List that will give you all the information you need as far as what codes mean.

http://myweb.accessus.net/~090/dtcobd1.html