99 Silverado 3/4 ton...sluggish

I have a '99 3/4 ton Silverado w/ a 6.0 V-8, when I hit the gas after the truck has been sitting for an extended period (over night), it is very sluggish at first. After a couple of minutes, it is almost normal...a little sluggish, but barely noticeable. I had it looked at and it is not the transmission. They stated it may be that I need to change the transmission fluid (truck has almost 70,000 miles) or there is a problem with the fuel pump. Any suggestions?