I strongly agree with these last comments. I competed at a high competitive level in both foil and epee, using a pistol grip in foil and at different times both a pistol and french in epee. It's accepted knowledge that in foil you can't really fence at the highest level using a french grip. The pistol grip (due to the leverage created by the grip's shape) is inherently a stronger grip, and that added power is more advantageous in foil, where flicks and absence of blade attacks (due to right of way) create a need for more hand speed and quicker hand motions during in-fighting. In epee, leverage and distance are greater factors, and the french grip is in many ways superior on these fronts, and it allows for a wide range of motion both laterally and front to back. You can actually choke up on the handle to create more of a "pistol grip" effect, or hold it back at the end (which requires A LOT of hand strength) to increase your length (great for shorter fencers, which I was) and your ability to release easier off your opponents' blade.
Now, if you hand is getting tired, you may indeed be gripping too hard, and you may also need to simply strengthen your hand more. With more practice, you'll find that the cramping diminishing. Grip strengthening devices are good. Also make sure to drink a lot of water (or gatorade) when fencing for a long period of time. In terms of grip tension, think of it like playing a musical instrument. A certain amount of strength is required, but TENSION is the enemy. If you try playing the piano while tense, your hands lock up and your finger speed disappears. So, as in some many other aspects of fencing, focused relaxation is key. Try not to obsess about your grip strength; just relax and fence. My coach used to say, "too much analysis leads to paralysis". Good luck!