1998 Jetta starting problem.

Help please. I have a 1998 VW Jetta Wolfsburg edition. Since July 2003 I have been having a problem where if I don't start my car for a day or two, when I go to start it, it cranks but won't run. The car has been to the dealer service department several times, they have changed the fuel pump four times and all I am being told, is maybe this or that is wrong. Final maybe: I need to have the tank replaced, a $700 job. I am totally frustrated since their mechanics don't seem to know what is wrong or how to correct the problem.
Has anyone ever had this problem before and if so how was it fixed? I'm a complete novice regarding repairs, but maybe if I know what is wrong, my son may be able to help.

Thanks in advance.

Did they state a reason why they focused on a tank replacement?

Did they state a reason why they focused on a tank replacement?

Yes. They said they think there might be contamination in the tank, although the fuel pump did not show any at all. They could not explain what kind of contamination they were talking about although I told them I put only gas in my car which is and always have been bought from a high volume gas station. BTW, this is a five speed standard and until I moved to Florida have never had a problem at all. In NY no matter how cold it got, it always started right up even after not being started for a week sometimes.