97 elantra fuel pump problem?

I have a 97Elantra wagon which has trouble starting about once a week. If I tap the underneath of the gastank while turning the key,it will usually fire up right away.
However, one day this week it wouldn't start at all. Strangly, the next morning it started right away once I tapped the gastank.
Has anyone else had this problem? Could it be a fuel pump problem? if so why doesn't it happen while the car is running?
Thanks for all responses. Dave (and confused).

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