I'm guessing it has surpassed it's useful age. I had some plastisol do the same thing after it was stored for 2 years. No matter how much I mixed it and how careful I was during the heating process it turned a reddish brown. I bought a new batch and solved the problem. I really don't know what the shelf life is, but I think the storage environment has a lot to do with the life of the plastisol. Mine had been stored where it could freeze in the garage.