I live in OC and was out in your neck of the woods a few weeks ago (Anza Borrego) and it was 110. My '81 ran cool as a cucumber even in the sand, it's completely stock as far as the cooling is concerned. Bottom line is, something is wrong if you are getting that hot. Try adding some WaterWetter, it did wonders for my hot running streetbike. I've put it in all my vehicles just for GP. The hole in your t-stat shouldn't have a negative effect because it should be open all the way once the engine warms. Maybe it isn't? Maybe you need to burp your system? Idle with the front elevated, radiator cap off for 15 to 20 minutes. Make sure your fan is spinning when the AC is on. Maybe it is getting hot and crapping out when it's needed most.