Unable to Solve Overheating 4000q

[LSmierciak@aol.com]

Has anyone ever experienced this mess?

My 86 4000csq is running very hot.  Naturally, the first thing I thought it
might be was the thermostat, so I put a new one in.  This didn't do anything.
 I then flushed the sytem and radiator using a good cleaner and descaler,
refilled it with 50/50 mixture of coolant and wather.......still hot.  I
changed the thermo switch in the radiator for the fan, nope.......Got a new
temperature sender and put that in.......nothing.

I thought too it might be the gauge, but the hot air blowing out of the vents
is hot enough to burn your hand if you leave it there long enough so I know
it's running hot.
Also, running on the open higway doesn't do too much to increase the cooling,
the only major drop occurs when I am able to run down a steep hill for an
extended period of time with the car in gear and the foot off the gas pedal.
 After about a mile of this, the gauge reads down where it should be
normally.

Does anyone out there have any ideas?

Thanks for your help! 
Larry Smierciak
86 4000csq