Newfieboy - Without looking at the vehicle, I would have to suggest that you start by checking your battery connections. With the headlights and the engine controls being essentially isolated from one another, that's about the only possibility I can come up with right now.
Lamar - A little more information on the engine would be helpful, but I would recommend checking the fuel pressure (Vortec engines should run about 60-65psi) and cleaning the throttle body and IAC passages.
Tamsinblevin - I can't say that I've ever heard them "off-board diagnostics" before. Anyway..."on-board diagnostics" refers to the computer's testing and monitoring of various systems, sensors, and actuators. When it detects a fault, it alerts the driver through a set of...
Bill - The main problem with answering your question is that GM temperature gauges are designed on a non-linear system. If you look at an analog GM temperature gauge, you notice that most of them are marked 100-200-260. It's not a simple question of "X ohms per degree". You should...
