Here's an update with what may be the final solution: when we changed the clutch on this, we noticed that one of the pins on the engine-to-car connectors was pushed halfway out, and had obviously not been making a secure contact. That was one of the pins for the TPS! We couldn't get the pin to go back into the connector, so we pulled it out of the connector housing on both sides, and plugged it together outside the housing. A little shrink tubing and it works again!