97 deville "a/c shut off to prevent engine damage"

My father has a '97 deville 150,000 mi. + last week it started to run hotter than normal while driving to work being on road for approx. 10 min. or so a/c shut down with error message "air conditioner shut off to prevent engine damage". He noticed temp. up to 260 deg. kept driving at a steady speed moments later dropped back down to approx 217 deg. Then on way home did same thing. I removed radiator to have checked @ radiator shop says has excellent flow. Repaled water pump and t-stat. Still does same thing. cooling fans appear to be working properly. Possible temp. sending unit? Not that familliar w/ Chevy product. Does not appear to have head gasket problem. Am a Ford fan myself. I just don't want to keep throwing parts at it. Thanx for any input. To me seams to be flow prob. or sensor issue.

Since you have checked or replaced the major items, I'd look into a head gasket problem. NAPA sells a tester kit for around $50 that detects the hydrocarbons from exhaust gases that would indicate a bad head gasket. I've used it before and it's not too hard to do.
Also, I'd check that the water pump belt is ok, and not slipping.

Check the outside temperature (on the dash). My outside temp sensor went out and would not let a lot of stuff work. Should cost around $20 and is located directly in front of the radiator.