how much RAM do you have in your computer? if its below 3 gigs it cant handle larger file/larger brushes so it tends to leave that bubly mark behind. you should use a smaller brush and work with it a bit slower.
Try to work with a smaller image size. Also helps if your PS is working with a Mac instead of a PC. Mac programs work better on Macs. I tried Painter on my Mac...which is a PC program...and I hated it soooo much.