Did you ever resolve this? My 04 Titan 4x4 does the same thing - but it seems to be only when I'm towing... I've replaced the thermostat, the waterpump, the fan clutch, checked that the electric fan/relay is working properly and verified that there is no air in the system. System isn't leaking or pressurising excessively to show symtoms of cracked head. What's strange is that the temp is perfectly fine when I'm not towing. If I'm towing (boat), it will spike from normal up to the 3/4 line before the H - and it does it QUICK! If I run the heater from the moment I leave the driveway while I am towing, the temp does perfectly fine - doesn't budge! If I turn the heater off, it quickly starts to climb (even when on the highway). I did notice that sometimes when I switch from AC to heater that it may take several minutes for heat to start blowing out of the vents - which is usually a sign of either a clogged heater core, or no water getting to the heater core??? I'm new to Titans so still trying to figure this thing out... Is there a possibility that a heater valve sticking shut could have something to do with it not getting full flow through the system, causing it to overheat when under a load??? I'm pretty skilled under the hood & have a mechanic in the family, but we're both to the point where we're scratchin our head here because the only way we can re-create the problem is by test driving with a heavy trailer behind us!! I'm ready to buy an extra electric fan with a manual switch for when I'm towing, adding on a tranny cooler and an oil cooler, just so I can pull my boat with the AC on (it's hot in TX!!!) Don't want to spend any more un-necessary $$$, so if you figured out what this issue was, I know at least 2 people that would be REALLY interested in knowing as well!!!