base timing keeps advancing by itself...

First thing is to verify the balancer ring has not slipped. It would be nice to know what engine as well. I have seen slants shear the distributor gear pin and it would run fine for a while then suddenly slip. Drove me nuts the first time I saw that because the ends of the pins were still in the gear so it visually looked fine.
I guess the same could happen to an intermediate shaft, although I cant say I have ever seen it happen.