jremington:
Take heart, you are not alone! At least 95% of the servo problems reported almost daily on this forum are due to inadequate power supplies.
Thanks for the link! I was looking all over the place for that info. Looking at the quite confusing DW01 datasheet,the chip has over current protection. It'll be that. I'm going to try using it to drive a PNP connected directly to the battery to avoid running the servo through that tiny protected NPN output. I just wish I hadn't hot-snotted everything before finding this issue!
Thank you again. I'm pretty sure I've got a grasp on the problem now thanks to you.