Intermittently cranks once but will not start

Hello,

We've replaced the fuel pump, the starter, and switched all the relays around. The first time you turn the key it cranks like it is going to turn over. But after about 5 seconds it still doesn't start, so I stop trying. Then if you try a second time, a few seconds later, it won't crank. All the lights come on but the starter just won't move. When it won't start, the fuel gauge is always on empty. I say intermittently because some days it starts right up. Although I have heard some grinding noises. And many times when it does start, it seems to buck. Like it hesitates to start, then kicks back on the brake pedal and really fires up the engine for a 1/4 second. I hope that's enough information.