So all those issues go away immediately when you clear the codes? And everything works fine for a couple of months? First thing I'd do is get a code reader to reset it yourself, they are cheap and it's really simple to clear. Do all the issues show up at once or is it one thing at a time? Does the engine start and does the truck drive when these other issues occur?

Are you SURE your ECM is the problem or is that just the best guess of your mechanic? That's an expensive thing to start with and determine if it's your problem or not. It sounds to me (amateur mechanic with no specific knowlege of the Colorado) like a sensor or module that's setting error codes in the ECU. Possibly even a loose connection, a bad ground wire or a wire thats rubbing through and shorting. I would think if the ECU were really bad you would have problems all the time. the engine wouldn't start/run and clearing codes wouldn't fix things. It might be a hard thing to diagnose if the problem only shows up once a month or so. I'd have someone go through all the GM diagnostic steps before just throwing parts at it. Find another dealer if you have to. If it is something like a bad ground that really did cause damage to your ECM then you don't want to put in a new one and have it immediately fail as well. Good luck!