Overheating problem, any clues why?

[71 RT]

I really don't know if this is a motor problem, or an electrical issue.  Any advice or help is much appreciated. 
The temp gauge on my rally dash show that my 440 is running hot.  When I drive 30-40 MPH it heats up SLOWLY to the top of the normal range, and then just keeps going very slowly to the "H".  It never goes past the H, just right up to it.  I assume it is running hot, but thought it might be the temp gauge is going bad.  Occasionally, after it warms up to near the "H", it will spike up, past the H.  This "Spike happens in about 1 second, and then comes back down to the H slowly (in about 3-4 seconds).  Here is my question.  Can the temperature really spike that fast, or is that a sign that the gauge is not working properly?  Is it possible that really hot water is suddenly hitting the temperature sending unit, causing it to spike like that?  I always thought these gauges were slow to changes.  All other gauges appear to be working properly.  I have replaced the sending unit.  I want to find out if it really is that hot.  I have a 22 inch radiator that was rebuilt, and have a shroud.  I also have a clutch style fan.  It is not spitting water on the floor when I stop the car.  I obviously do not want to hurt the motor or heads.  If I want to hook up an aftermarket temp gauge, what is the easiest way to do it, without hurting the water pump, radiator, etc?  I just want to confirm that it really is running as hot as it appears. 
I have even thought that maybe I could get a voltage meter reading on the back if the temp gauge, but have no idea how much it is supposed to read.  Could it be a radiator hose collapsing?  Is there supposed to be a spring inside the lower radiator hose to keep it from doing this?  Any thoughts or advice is appreciated.

[Charger-Bodie]

have someone check it with a lazer temp gun. just point it right at were the sending unit screws in and find out what its really running for temp.

[Challenger340]

Any other history here ?
New build ?
or, if an old build, did U change anything lately ? ?

[Steve P.]

Check coolant level.  (COLD)....

Check for leaks.

Check cap.

While still cold, remove the rad. cap and have someone else start the motor. Watch to see if you have coolant flowing across the rad.  If nothing right away, stand back and watch to see that the T-Stat DOES open. It should be flowing like hell when the stat opens. Just make sure you are not going to get nailed by the smoking hot water when it does open..

NOTE: If coolant is flowing from initial start up cold, shut the motor off. Drain coolant down about a gallon and replace the T-Stat and gasket. Cold flow is either 1) no T-stat  or   2) stuck open T-stat. Neither are good.


If this all checks out good we will need more info. for sure.. A/C or not? Any recent changes? Fresh build? More.......

Let us know..