Man I have the same engine in my SDL and I've been recently experiencing wacky engine temp syndrome. I've been searching and following a couple of threads here and they all seem to relate to cracked cylinder heads! Now of course I don't know but you situation seems like it fits. You said you had your head off to do a valve job, was you head checked at that time thoroughly for straightness and cracks? If the head wasn't right the new head gasket could fail and it seems like that situation is a precursor to eventual head failure from thermal stress. I hope not but its something to consider. I'm waiting for a tool to pull my head bolts and then the head for a look at the gasket and the head area between the valves. You can do a search for this 603 motor and cylinder heads to find an ongoing head failure/replacement discussion, Good Luck!