Have you checked the head gasket? I know when my HG blew on my '91 T/A that it drove perfectly fine without a miss but would overheat within a minute or two a cranking. If the HG is blown close to the sending unit it will pick up the exhaust gases and give you a false reading. Check your oil for water and vice versus.

'86 Toyo 4wd
'04 600RR