we had a blazer in the shop the other day that was having a similar problem. It ended up being the positive battery cable going from the battery to the starter. It was shorting out on the frame or something. I cant remember. On the starter positive wire there should be a tab to make sure it is connected a certain way. If is not exactly right like the the one we had it might cause the cable to twist funny and rub against something metal.

 

I would suggest carrying a multimeter with you while you drive it. The next time it happens, leave the key on, measure the voltage between the negative POST on the battery and the terminal. if you see 12 volts, its a bad connection. Next measure from the post to the engine block. If you see 12 volts, you have a poor connection or a bad cable. Next check the ground cable to the body in the same way. You should show NO voltage (Or just millivolts at most). This will tell you if its a grounding issue. Good luck.