If this condition start just after having the stat and hoses replaced I think either the stat is in backwards or upside down. The stat should have a small hole at the top so coolant passes through to bleed out air and if it is upside down the you trap air in the head and then the stat doesn't know when to open. It was 100 here saturday and my car ran in the 80s with the A/C on with no problem.

__________________
1983 300SD
200000miles