Well it still happened with the switch over valve dissconnected. I ohm tested the coolant temp sensor and it was a bit out of spec with the engine cold, but in the range when hot. I'm leaning towards O2 sensor, but I think I'll unplug my IAC motor force a high idle of course but see if it happens again. If it does then I'll know the IAC has no part of it.

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1989 560sec with a few mods.