1) I would recommend that you guys go get one of those $29.00 code scanners and plug it in the truck to see what the trouble code is. Even though the 'Check Engine' light goes off, the EEC (computer) will store any codes that appear and are then self-cleared due to the situation that caused the code having 'fixed itself'. The stored code will tell you what's going on, or at least it's a start as to what's going on.
2) If you haven't changed your thermostat, CHANGE IT NOW! Explorers are highly intolerant of stuck thermostats, especially ones that stick closed and cause overheating. There was a horror story on the old message board of an Explorer that was less than 2,000 miles out of the 50k powertrain warranty. She had just bought the truck from a 'reputable' Ford dealer. Well, the thermostat stuck, causing overheating. At least one head cracked, even before she got to the shoulder of the road, and now she has a new engine, all at her expense. There is no delay between the point when the gauge says HOT and engine damage time. FoMoCo basically told her to go fly a kite.