[THEFERMANATOR]

Impeller sounds about right, or a problem with the check balls on the T-stat's not sealing. Your alpha outdrives impeller doesn't move as much water into the block at idle as the circulating pump will pump out when the T-stat opens. So the temp will climb up as the water is pumped out and air gets sucked back through the manifoilds. Fairly common issue with the ALPHA's. Do the impeller first to make sure that is right, wouldn't be a bad isea to do the whole kit. If that doesn't do it get a replacement check ball kit. I went through this on that SEARAY last year, ran dead on at 160 when running but would climb up to 180 at low idle. Come up to 800+ RPM's and it would drop back instantly.