If it's now immediately stumbling and going to a high idle condition right after a cold start then that changes things. It's suggestive that the ECU either can't control idle or doesn't know it should be trying to control idle. Perhaps your initial TPS thought is valid.

For idling and stalling issues, I always like to start with the basics:

Start by making sure the TPS electrical connector is fully seated and you can hear the click of the TPS just as the throttle is beginning to open and again just as it closes. Following that, verify TPS operation and sensing using DTM 2 as I first suggested. It should not be set on the hairy edge where you barely need to encourage it to get a click. There are specs as to the gap at the throttle stop screw for the click, how to properly adjust the TPS orientation (it's different for Bosch and VDO TPS switches), also adjustment specs for the throttle spindle linkage rod and throttle cable rest position that can all affect idle mode sensing.

Make sure the IAC valve electrical connector and hoses are fully seated. To verify that it's capable of being controlled, use DTM 3 to exercise it, listening for the indicative tick-tock or using your hand to feel for the vibration of the piston rocking back and forth hitting end of travel. The IAC valve can also be removed for inspection and a light cleaning if needed, just don't soak it so that fluid gets in the motor cavity.

If the throttle body throat hasn't been cleaned in recent memory then do yourself a favour and remove it to the bench for a thorough cleaning of the throttle plate edge and adjacent throat area, also making sure the vacuum orifices are clear and the PCV hoses aren't plugged, also the flame trap (non-turbo) as you're in the same area. As long as you remove it off the manifold slowly and carefully, the gasket normally survives -you can always make a temporary one out of cereal box board if needed. At the same time, if there's any chance someone has altered adjustment of the throttle plate stop screw (as might be evidenced by missing or broken factory paint indicator on the screw head), you can re-adjust the throttle stop position (use the preferred 1/4 turn method past when it just touches and starts to open before locking it, which you can find described elsewhwere here).

With regard to the fuel trim trouble code, it may or may not be a red herring. Sometimes the Check Engine light blinks on and trouble codes are set when the engine is rocking around during a stall, or more rarely when the engine races and quickly dies. Always clear the trouble codes and wait for them to come back before getting too excited about condemning things like the O2 sensor. There are additional tests you can do with a meter to check the voltage range of the O2 sensor and also the response (a slow response is indicative of fouling on the sensor head and a possibly failing heating element).

Vacuum leaks can easily be behind fuel trim issues, so make sure that all air hoses on the intake side are fully seated, not split and that the accordioned air supply tube doesn't have a minor split in it, such as underneath.

Now having said all that, if it can start from cold without stumbling, and it's more a matter of a steadily increasing idle while the engine is warming up heading towards a racing condition, like heading upwards of 1500 rpm, then the ECT sensor goes to the top of the suspect list. See the FAQ for more information.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now