98 GMC Jimmy - Ignition key won't release

My 98 Jimmy has an intermittant problem - the ignition cylinder will not rotate from RUN to the OFF position.
I confirmed that the key cylinder solenoid works fine, and that the
Transmission Controller (under the shifter) properly closes the circuit (pins C and F) when in the park position. Usually the solenoid properly pulls in when in Park, sometimes it does not.
I can't reproduce the problem right now - has anyone seen this problem before?
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