Ok guys, I believe I have a solution!!!!
I posted about my problem months ago hope someone here could find out but finally got sick of it can call Apple support. this seems to have solved my issue in photohop 5.1 running on my 27" imac. hope it works for you all too. The Apple tech was great by the way!!
first restart your computer in safe mode:
restart the computer and when you hear the chime, hold SHIFT until you see the apple icon.
launch the program that you're experiencing the problem with and try it. in my case it was Photoshop in th brush mode. It worked for me in the safe mode so we figured it was related to something else besides Mountain Lion and Photoshop.
Next restart the computer and once it's up, go to System preferences / User & Group then click the lock to unlock it. then go to to Login item. here do not just click on the BOB but click on the NAME of the item and click minus on anything that may be problematic. start with things you don't know if you don't want to remove everything. I removed Dropbox, Google Drive, VMFusion, a fe Adobe items and left only mail and calendar alone.
retart the computer and lauched Photoshop and Voila! tested out the brush tool and it was golden.
good luck you guys!