Problems starting 06 xterra

I have an '06 Xterra and I recently experienced sputtering and my fuel light was on so I thought I was about to run out of gas. I pulled into the gas station and got about a half tank. When I tried to start the car, it would just sputter and idle very roughly. I had the car towed home and when I tried to start it again, I had to crank it a lot to get it to even sputter. My fuel pump activates when I turn the ignition on. A friend told me to open the EVAP SERVICE PORT, it looks like a tire valve, and when I did that, the car started right up. The next morning when I tried starting it again, the car wouldn't start unless I opened and relieved what ever pressure had built up through the EVAP SERVICE PORT. Could this be a problem with the EVAP cartridge or with the Purge Control Valve?