Rob, ignore "percolating" as it refers to the liquid cooling. This is only to describe the sound it makes as it coughs and chokes itself off. Sometimes there is a backfire. I have not seen an overtemp condition by guage or by boil over, the fluid level is good. If a bad temp sensor could be the culprit, that is a possibilty I guess. Otherwise I'm stumped, thanks for hanging in there.