To me, it seems to be a virtual memory saturation problem. This is the kind of problem that can happen when you're working on big files. I was working with 16go ram before and had that problem from time to time,and with other softwares too. I tweaked my virtual memory setting (when ram is full, it t...

I found out what the problem is for the flat glitch at the begining of a stroke : it comes from the double-click timer.
In the tablet setting, select your pen and paintstorm, and pull the slider to the left, to off. Worked for me.

Having kinda the same issue but with the start of the strokes. I have to put the lead, wait half a second, then stroke, it's just to annoying a problem to use paintstorm now. Problem appeared a few days ago after my wacom driver update (I'm using a cintiq), but it works fine on other softs like clip...