95 Gmc pickup - runs poorly at startup

I have a 1995 GMC k1500 extended cab 4 x 4 with a 350 (5.7 litre) TBI with 190,000 miles. On very humid or cold days when I first start the truck it runs very poorly under load, with an ocasional backfire. After it warms up it runs great (5-10 minutes later). All basic ignition systems are new or good ( wires, plugs, rotor, cap, etc...). There is no check engine light. I have checked the M.A.P. sensor and it falls within normall parameters. 5 volts exactically while not running and 1.2 volts at idle. Could it be the ecm itself or what else may cause the missfiring at first startup.