Root beer kegging problem

I am having trouble with my corny of root beer. It still has around 1/3 of a tank. I have the pressure set at 40 PSI and have around 30-35 ft of line (3/16 ID). It worked reasonably well for the first 2/3 of the corny but now it seems to come out in spurts of foam. I have tried bleeding the pressure and running it out at 30 PSI but I still get intermittent bursts of liquid (mostly foam). Anyone know what's up? Thanks very much.

I wonder if it's overcarbed now? Once you get some, and the foam dies down, how is it?

Well it seems a little bubbly but mostly flat.

What happens if you keep the carb pressure at 40 psi, but drop the serving pressure way down? Like just enough pressure to push out the rootbeer?