Could be a few things. The earlier model years had a problem with converter drain back if they sat for more than a few days. Letting them idle for a minute or two usually took care of that problem. If your fluid level is low and it's cold it can do the same thing. The other possibility is the NSBU switch. Look on the driver side of the transmission towards the rear and see if you have a black or tan switch. The original black switches will sometimes go bad if you drive around in wet, salty conditions. The tan switch is an updgrade.
Last week had the same problem...it was the tcm (which is covered under the emissions warranty). Originally they gave us a 1050 bill, (as we didn't have extended warranty) but there is a bunch of things covered and they are listed in the warranty supp that came with your truck. Good luck.
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