I have an '06 F250 6.0 with 122k miles. The heat hasn't worked in it for a year or so. It'll blow cold air, but switch from vent to defrost to floor just fine. About once every two-three weeks it'll blow hot air for a few minutes then switch back to cold. It doesn't matter if I'm stopped or going 80 mph. It just switches to hot randomly and then its gone and I'm back to being cold. Haha. This morning on my way to work it was -2 out and 20 miles into my commute the "Check Gauges" alert beeped at me and my coolant gauge was up in the red. I took my exit and about 1 miles later it was fine, back to normal. Drive 5 more miles and it freaks out again. I don't know if the problems are related. The coolant level is fine with no leaks. When I bought it in July of 2012 I put an SCT tuner on it with custom tunes and got the EGR delete done and my turbo rebuilt. I run it in the fuel saver mode 90% of the time. It's in that tune now. Someone said it may be the thermostat. Ford will look at it for free and a thermostat install/replace will run about $250. The local diesel shop wants $125 diagnostic fee and an est of $575 to repair... Any ideas?