Just purchased an '05 F-250 with a 6.0L Diesel. Test drove around town but never got it over about 50 mph. After signing all the paperwork and heading home (about 20 miles down the road) "check guages" appears on panel. The coolant temp guage is redlined so I pull the truck to the shoulder to check the coolant level. Hardly any coolant showing in the reservoir. I cracked the cap on the reservoir and its pressured up. As the pressure bleeds the reservoir fills up with coolant and slowly recedes. Took about 10 minutes to completely bleed all the pressure to take the reservoir cap off. Screwed the cap back on and fired it up, guage had returned to showing normal operating temp but another 15 or so miles down the road it overheated again. Do I have head gasket trouble? Would appreciate and tips/advice, thanks.