The Progressbar widget has been known for a while to have memory gain problems, but from a fork, I don't see why the Progressbar would cause this. Can you make a minimal example, that actually shows the code that produces the error.

I get a clue from your problem description

queries certain nodes an displays a graph and is a bit time consuming

I'm thinking your long process is somehow hogging the eventloop and won't let the progressbar update properly. Maybe try a different widget. Can you substitute for the Progressbar, a simple Label with a counter with a textvariable, and see if that causes problems?

It is quite easy to make your own progressbar on the canvas, if that workaround is succesful.

why does it happen intermittently

It seems that depending on the load of your computer, the eventloop sometimes dosn't have enough to do all it's neccessary tasks, before being forced to skip some. Just a guess.