I feel i need to note that i found this post from an earlier problem on this site and thought it was very informative:

Posted by zl700 (My Page) on Tue, May 13, 08 at 9:23
"It�s probably caused by the pressure balancing spool in your valve attempting to adjust for fluctuations in the hot and cold supply pipes. This can be difficult to fix easily without figuring where the flow and pressure differences are happening. In a perfect world both pipes feeding the valve are of equal lengths, have the same number of valves and fittings and so on, which we know is impossible.
If you have stop valves on or near the shower valves, try adjusting them, usually the culprit is the cold. Also try removing the flow restrictor on the shower head which very often increases the flow curing the problem."

Do I need to adjust the balancing spool (or replace). I'm not sure if there are adjustable valves in our crawl for the hot and cold water lines in our shower, so that isnt an option. I do want to note that my hot water heater is upstairs and the problem is downstairs...maybe gravity is causing excessive pressure in the HW line? I know it isnt "water hammer"....i feel it's mainly due to pressure differences through a restricted space. Anyway, i'm usually pretty good in figuring this stuff out, but any help would be great.

Thanks!