Pewter must be kept very close to it's melting point. If it is overheated, it will do exactly what you described-- your description is a good one. Pewter will turn all kinds of colors and evolve and aggressively-forming frothy or foamy dross when kept too hot.
If you mix much pewter into your lead, you must
turn the temp way down. Adding pewter will drop the melting point of the lead alloy dramatically, and you should only pour at about 50*F above the melting point. If you keep it at a temp suitable for pouring pure lead, it will rapidly turn into a pot full of frothy, clumpy dross. At that point it becomes difficult to deal with, and fluxing still results in a lot of metal loss.
Keep the temp low. Turning up the heat only makes it worse.
I find a lot of pewter at yard sales, and I buy all I can find cheap. It works very well as an alloying component, but one must treat it as it wants. Only as hot as necessary, and not hotter is the goal.
Hope this helps, good luck!