Okay, so your car is hard to start, but is it running nicely without any problems? If the vacuum gets to big the fuelpump can't manage to pump the fuel and the car will stop. First time I found out that I had vacuum on my tank was because the car keeped stopping. (It was a Jag of course) And when I checked the fuel cap, I had a major bil vacuum and thought that was the problem, but it wasn't. Usually as said before some cars have vacuum on the tank. One thing you can try first, is: When you're tank is almost empty, take out the fuel cap and don't put it back on. Drive a little, stop and start the engine. Is it ok now? If it is you can just drill a hole in the fuel cap.
And obout the Jag... After 6 months we found out that there was a little screw in the tank which got sucked in the pump. After a little while, the vacuum in the pump was getting smaller, and the screw came back out in the tank... Funny, huh?