I think, it is time to replace the cover. Nothing much can be done to maintain it. If you have decided to buy a new spa cover, then better go for MARINE grade vinyl cover. Also the cover should have closed cell virgin foam insulation. I had purchased a similar cover from www.thecoverguy.com and it works well for my spa. You can check out this website for specifications.

We have a couple cabin rental and they have hot tubs. I have tried the foam drying out before and it did not work. I have also tried a few different sources for hot tub covers and the best by far has been hottubworks.com. They sell an option to seal the foam against water and it works, lasted allot longer than the others. Hope that helps...

Here is a link that might be useful: spa covers