It looks similar to this issue, but you are getting it with a single touch instead of with a multitouch. I get EXACTLY the same thing if I use a second tap while drawing a line. Try "multitouch visualizer 2" from the market and see if it registers one tap that is on all the time or if you can actually get the full 4 touches at once. I doubt it will be insighful, but it can't hurt--and if you have an "always on" (perhaps in the button area below the screen) then you know the digitizer is bad.
If it is the same issue, try restarting and give it time. Mine has gone from happening often to happening about 1% of the time. It is software and can be "corrected" by either time or a restart. It is WORST on the first boot after flashing something (even JUST a stock kernel). I have absolutely no idea why.
Good luck. I hate this junk. My Evo 4G never had this sort of glitchy garbage, but, whatever it is, it is likely due to HTC's glitchy kernel!