This really sounds to be more of a medical problem so normally I would be reluctant to give advice and suggest you visit your vet, but you have already done that.

If the worming tablet helps, have you spoken to the vet about administering a low dosage wormer on a more regualr basis (DO NOT DO THIS WITHOUT TALKING TO YOUR VET FIRST!).

I always think dry food is a bit more sensitive on a cats tummy then wet food, have you talked to your vet about your cats diet?

If you have had little or no sucess with this vet, you can get a second opinion, either at the same vets practice, or a completly different one.