86 CGT rough at low rpm

["Englert, Rick" <RickE@triadperform.com>]

Hi!
  My 86 CGT has started acting up a little lately.  It's just a little
annoying at this stage, but I'd like to know where I should start looking to
fix it.  Here's the problem, while accelerating at low rpm, (below 2000) it
seems to only be running on 3 or 4 cylinders, and then as soon as rpms hit
around 2000, the rest kick in and everything's great.  It only does it while
accelerating, with or without load - ie if it's in neutral and I just rev
it, it does the same thing.  It idles smoothly though (well, not brand-new
car smooth, but pretty close).  Throttle position doesn't make a difference
- stumbles just the same at part throttle as if I floor it (which, of
course, I don't do if it's in gear below 2000 rpm)  I have run two bottles
of injector cleaner through the system to no avail.
Don't know if it's related or not, but every few months the fan doesn't shut
off for a day or so, like the temp sensor that controls it gets stuck for a
few hours.  The car is stock, original and has 205,000 miles.  Oh, it did
get a new head gasket three years ago.  Thanks for any suggestions!
Rick