Cooking "steak tips" on the grill - how to avoid making them tough?
Tough meat boils down to 3 factors: 

The way it's cooked, the way it's cut and the quality of the meat.

I know this is an old question, but next time you encounter tough meat on the grill, perhaps try pan frying it. If its still tough as rocks, I would question the quality of perhaps that batch of meat, provided it was cut properly.