95 ECM Code 25

My wife's 95 Corolla (1.8/auto) recently turned 150K so I did quite a bit of maintenance. Since then the check engine light has come on with a code 25 (lean mixture) stored. I have cleaned the electrical connectors of the stuff I disconnected (IAT sensor/coolant temp sensor/connector near distributor cap). When I reset the computer the light can take quite a while to come back on (several hours of operation). Was hoping someone might have an idea...

I have checked the IAT sensor/connector and they appear fine.
The coolant temp sensor looks like it _might_ be bad...
Not sure how to measurer voltage from coolant temp sensor connecter.
(Where do I ground the voltmeter? It is a single wire connector.)

The car starts and runs just fine. I did replace the thermostat and the upper and lower radiator hoses when I did all the maintenance. (And obviously flushed the system and refilled it w/new coolant.) Also replaced the distributor cap and rotor, fuel and air filters, and changed the spark plugs/wires and tranny fluid.

Anyone else seen anything similar? I would rather fix it myself than take it to Toyota and spend $$$$ to have them figure it out and fix it...

Any help greatly appreciated!
John