Silverado fuel problem

I have a 1999 chevy silverado. Here is my story. I left the lights on overnight and the battery drained. When I went to jump start it, the engine turned over but it wouldnt start. I was able to determine that I wasn't getting fuel to the injectors. We tapped on the gas tank while cranking and it didn't help. Then, I had someone tap on the fuel pump relay and it started right up. I went right to the dealer and bought a new relay. Three days later, it happened again in a grocery store parking lot. Logic told me it wasn't the relay but I was so desperate that I asked a guy to tap on the relay while I tried to start it. It didn't start right up, but it sputtered a little, which was better than it was doing. The second try and it started. I replaced the fuel filter and held my breath. It ran for about 3 weeks until today. I have tried tapping the gas tank and the relay. Nothing. Before I spend $700 to replace the fuel pump, I want to be sure there isn't something simple that I might be overlooking.

check the battery connections. the delco battery has known to go bad.