This problem is now confirmed on the GoMac web page. Sam Caughron (of Proteron) writes: "It has yet to be determined which program is the cause of the problem. If the problem is with GoMac, there will be a fix available sometime early this coming week." (Latest word is that a forthcoming update to GoMac will fix this.)
Meanwhile, I have gotten an assortment of other bug reports related to Speed Doubler, but not enough confirming reports to want to post them yet.