I think I've read somewhere on Usenet that SpeedDoubler starts a background application and this application has problem when the Finder sends it a request to quit. This is caused by the increased memory requirement with 8.6. I know that the fix is to modify some resources in SpeedDoubler with ResEdit, but I unfortunately don't remember which exactly. I think it's the SIZE resource. The memory needs to be increased by about 300K. Do a search on Deja-News, in comp.sys.mac.system and you should find some info there.