[V8] Overheating - revisited

[Scott Baumann / PTS Power]

Howdy, gang!
230K on the clock, and running as strong as ever.  My only problem is that once the outside temp is above 80-85 degrees, I DO NOT DARE stop in a drive-through, and PRAY not to hit a traffic jam!  The reason being that my 1990 V8Q will run all day long and as hard as I ask it, maintaining a sub mid-point temp the whole time.  10 minutes stopped, however, has my temp gauge skyrocketing.  Shutting down the A/C and revving up the engine in neutral helps to being the temp down a bit, but if it lasts more than a few minutes, I am dead in the water with what seems to be a vapor lock.  No amount of cranking will bring it back to life, until an adequate amount of cooling time has elapsed.  Am I fighting a failing T-stat here?  All fans operate as they should (checked all of that out).  This is my third summer I am facing with these issues.  I am seriously considering parking this V8 for the summer and only using it for a cold weather car (85% of days here in Wisconsin anyway).  Any ideas??

Regards,

Scott B in WI
1990 V8Q
2000 A8Q
2003 Navigator (avoid these like anything)