Uh oh....

[SouthJ1]

No codes Triathlete,I'm gonna check around the radiator supports tomorrow. And I've only pressure washed under the hood 3 or 4 times so far

[Mile High VX]

Had this exact same issue with my '69 Z28...the wire that feeds the tach had worn out at the firewall and was shorting out.

Definatly sounds like a short or loose connection associated with spark. Another thought would be that the gear selector could have a short as well. Hit a bump and it gets out of alignment so you have to put it in Park to get it "reset".

That's my WAG....