I agree that if the moisture is under the plastic, the plastic is doing it's job. However, I disagree with the application of insulation under the floor joist, rather, I would suggest insulating the foundation perimeter, either with fiberglass or with foam, sealing up any ventilation in such areas. You may also wish to add an electric heat unit in your crawlspace (Your floors are only as warm as the space beneath them. This is why mobile home floors are cold.). The reason for this is that by insulating the floors, you effectively isolating the crawlspace from you heat source, making it cooler and opening the way for more condensation. Plus, many building codes require crawlspace ventilation only if the floor is insulated & the crawlspace is outside the thermal envelope. The result is allowing hot, humid summer air in in the summer and making the crawlspace cold in the winter, thus making the floor cold.
I have used this method to solve similar problems.