Not seeing how you bowl, I can only guess. But my suggestions are:
1: Relax - let gravity be your friend, both on the entire backswing and the entire follow thru. Muscling the ball/guiding the ball thru the entire swing is one possible reason for what you're going thru. Think of your arm as a pendulum of a grandfather clock (if you're familiar w/that analogy). The less arm muscle you put into the shot, the more accurate and the longer you'll be able to bowl w/out fatigue.
2: According to PB3, the only real workout you may do would increase wrist and forearm strength -- squeezing a tennis ball a few dozen times a day. Not to kill it and mash it flat, but just to increase grip strength.
3: And if 1&2 fails, perhaps a bit more potassium and calcium in your diet. Charlie horses are a type of cramp, and natural potassium (bananas, spinich, etc) along w/a bit more calcium (to help metabolize the potassium) will help combat it.
Edited on 8/29/2007 1:11 PM