The biggest problem I've come across with the 3.5 is the vvt cam phaser getting stuck in the advance position. This usually happens when the vvt oil control solenoid screens get clogged, come apart, or the valve fails. If it doesn't get replaced right away the phaser can get "locked" into a position that makes it run bad, then it's time for expensive work. Generally, if the oil is kept clean and topped off the problem is less likely to happen. Other then that, thermostats are a common failure (stuck open) and I've dealt with a few Colorado's with wiring issues near the fuse block under the hood causing ignition problems (wires breaking inside the harness, almost always a wire going to an ignition coil for some reason).
Not really sure what to look for on the rest of the truck besides the usual stuff.
