New problem starting when cold

[kc5deb]

I've noticed the past 2 mornings, I've tried to start my truck, and when you try to start it, it trys to turn over, and does a "chug...chug...chug chug.....chug chug *start*". It's like it's missing on a few cylinders when trying to start, but, once it turns over, it runs great. The last 2 mornings have been really humid, if that might have something to do with it.

 

Any hints as to where to start? (no pun intneded, I swear)

 

BTW, I just went outside to start it after it's been setting for about 2 1/2 hours, and it started up fine...

 

My truck's a 2002 GMC 2500 4x4 with the 6.0L and only 37,000 miles.