I have a 1993 king cobra with a 454 that is overheating. It was running hot at idle and would cool down at 20 to 25 just on plane and overheat again above 3500 rpm's. melted my through hull exhaust line on the driver side when it was reading around 160 on the gauge. Just had the feed pump impeller replaced and ran fine for the whole weekend. Sat for a couple weeks and started acting up. Impeller in feed pump still looks good. Any ideas. Thanks