Patbruce,

Does it have leaves or just the twining yellow strings? It sounds like it could be a species of dodder which is a parasitic non-photosynthesizing plant. It's related to morning glories, but don't let that stop you; keep pulling it as soon as it appears. Some people advocate throwing away any affected plants because it can keep coming back from it's roots and spreads easily to other containers by it's seed.

Here is a link that might be useful:

Yes, sounds like Cuscuta sp.