Was this ever fixed?
I had the same issue when I flushed.
I ended up figuring out it was an air pocket. Squeezing the hoses to remove air didnt help (as suggested online)
I had to re-flush and problem was solved. At some point during the first flush I must of done something that let a little air in before/after I had flushed with water.
If you get a new thermostat make sure the temp range is the same as the one you had before. Ive gotten slightly different ones and I dont know if it changes anything (other than temp it opens) but it bothers the crap out of me if my temp gauge doesnt always do the same thing.