I'm with Don on checking the condenser's electric fan. I remember this happening when visiting with you, but forgot to pursue it further by checking fan operation at extended idle and also looking at the fan fuse/relay. Not an uncommon thing to happen, even the fan's connectors can get loose or pinched. The fact it cools fine while moving is a good indicator this may be the problem. Had this happen on one of the Merkurs, easy fix once you confirm.