[Spridgets] overheating or is this normal?

[ryan.kubanoff at comcast.net]

First summer with the midgit.  It is 98+ today here in PA.  i parked her in the sun for a few hours and when i went to get in the temp gauge read 1950 ( i have a new 1800 thermostat).  i drove for about 5 minutes and stopped at a  light and while waiting the temp shot up to 212-2150.  i pulled over opened the heater valve and drove.  when I was moving the temp went back down to 1950 but everytime i stopped it went back up to 2120 territory.  I have water, 34 year old gunk, and remnants of a recent cooling flush in the cooling system.  
Is this something to be concerned about or am I just being overly concerned.
My plan is to drain flush and refill the system with fresh water. any other suggestions.

previously when i was concerned about the cooling system and it turned out to be a blockage in the rear engine, but thanks to frank and the rest of the OSHIT crowd that went away and now this?  But this doesn't seem like a blockage issue just poor heat dispersion.