I have a 2003 Hummer H2 with the GM 6.0. When the rpms go up, mostly with cruize on up hills or towing, the engine temp rises. It will start to drop when I let up on the accerlator. I'm thinking about changing the thermostat, but that is not the normal sign of thermostat going bad. Just worring if it is doing damage to the engine. Recently I spend more time watching the temp gauge. It doesn't happen when at idol, but when drive it will rise to about 210 then drop back. Doesn't do it all the time but from time to time, usually longer trips. Half or more. I only drive about 60 miles a week on average. The check engine light will come on from time to time also, never stays on but will do it on longer trips or when in a tow.