Mercury 5 horse mechanical issue

The outboard starts right up and stalls out after about 1 1/2 hours of operation running at 1 -1.3 mph. It won't start up unless it cools down. The spark plugs have been changed 2 times since last August. Every time i've changed them, the plug is pretty dirty.
Does carbon build up because i'm running it so slow? Any suggestions?

Thanks,

Chuck

Mercury 5 horse mechanical issue