1998 Chevy Truck Truck won't start, then does?

Test the coolant temperature sensor and the fuel pressure-

When you changed the fuel pump in 06 did you change the fuel pump chassy wiring harness connector?
Do you have a PASSLOCK system on your truck? PASSLOCK is a factory passive security feature--If you have it there will be a security light that lights up breifly when you turn the key.

I am assuming that you tested the coolant sensor and the fuel pressure and didn't find a problem. Â Â Â Â Â Â A Passlock system is not a security system, it is an anti-theft system. A security light will light up in the dash board if your truck has it--I'm pretty sure it does. Anyway--the passlock system relies on a certain cranking resistance, in the ignition switch circuit, to start your engine. The ignition switch has a resistor in it that provides this resistance -and- this resistance value is unique to your truck. Sometimes this resistor goes bad--especialy when there is an aftermarket security system installed--because (usually) an aftermarket security system is spliced into the passlock system in some way. A loose connection or something like that, in your security system can cause the problem your having. Â Â The fuel pump connector has been a problem for quite some time. The fact that the engine has run fine for two years does not always rule out this connector problem, but it is less likely. The connector problem is--loose pins inside of the connector. Another problem area is the ground for the fuel pump. The ground corrodes where it connects to the frame. This very common and in my opinion one of the most overlooked fuel pump problems. This ground connects to the frame cross member, just before the gas tank. It's a black wire coming out of the wire harness for the fuel pump--just before the tank. Clean up this connection, or better yet, cut it off, crimp a new connector on the wire, and connect it to the frame with a stainless steel nut and bolt.