My beautiful blue and rust 83 Suburban 6.2 diesel won't start when warm. It will start fine if it's stone cold, and fine if it's fully warmed up, but if I stop and leave it for 20 minutes or so, the glow plugs don't come on and it won't start. If I sit and wait until the temp gauge gets down to about 100 the glow plugs finally kick in and then it fires right up. New glow plug controller, new glow plugs. It seems if I'm below about 150 on the temp gauge I have this problem. I'm new to diesels and am stumped. I suppose I could bypass the controller with a manual switch, but I'd like to figure out what's going on here. Any ideas?