I'll try the over-ride switch idea and see if that helps. I have done this a few times before on Ford solenoids when neutral safety switch was bad but with a pushbutton switch.

I got to drive the car in to work this morning (about 9mls@55mph & 4mls@35-45mph). During my 12 minute drive here's what happened:
The first 3mls temp gauge stayed @ 40
next 5mls temp gauge read 80
next 2mls @ 85
next 1ml @ 90
next 1ml @ 100
during the last mile temp shot up to 118 and stayed there. Turned defroster on to try to cycle some of the heat and only blew cold from all vents. Also tried other buttons and temp controller...all air blew cool/cold from all vents. He had this same problem.

"You still have not verified that the HIGH FAN comes On at 105..if the sensors is out of wack, it will still work, but it may be coming on late [ like 115C]..see why I keep asking that question?"
The HIGH fan came on around 110-115 (radio off/windows down...I could hear it).

"Many guys also bridge that sensor [the blu one] with a resistor of proper value to bring the cut-in of the high fan down to around 100C , vs the factory 105C..that is a simple , .99 cent modification that works well on that system."
What is the proper value of resistor needed to do this? I'm willing to try just about anything at this point.

"I am inclined to think you have a sticking thermo just b/c of the intermittant character of the complaint. [ or air pocket] Are you using cheapo thermos ??"
We have tried two different 194 degree thermos and current one in is 180 degree all three were from NAPA, all are Stant brand. I've never had this problem before with their thermos. One thing I did notice when I replaced the original thermo...there was no o-ring gasket and the housing was sealed with Red RTV. I am using the o-ring gasket with the thermos I have been installing but the thermos all have a seep hole in them (so, I don't feel this has much to do with out problem).

Viscous fan seems to be okay (from the others posts I've read here)...fan spins freely, at least 3 revs, when it is cool. When its hot/overheated I can't even get a quarter rev, it has alot of resistance.