I have a 1998 GMC Sierra 4x4 Z71 that I purchased about a year ago.  
It has 287k on the clock, but is in incredible condition.  Runs like
a top, looks like a new truck and is clean as a whistle.  However,
the cruise control has recently begun acting up.  It works sometimes,
but not very well.  It will engage, but not hold speed, but then if
I'm holding down the gas pedal, it will sometimes decide to work for
a short period of time before slowing down or not working.  I have a
Haynes manual for it, but that's totally useless.  All it shows is a
cruise control wiring diagram and the fuse location (#6).  I checked
the cables and the connectors to the controller, and all seems to be
in order.

I did recently replace the steering column turn signal switch, as it
had a bad washer portion, but the cruise problem was prior to that
and the new switch is working fine.

Any ideas on where or how to start looking for an answer before I
have to take it into the dealer and get fleeced, or just live without
a CC?

Thanks