Knots in spectra are notorious for slipping.. well not necessarily slipping, but increasing in length as they set.
Spectra also creeps. But there are different varieties that people all refer to as spectra and they aren't the same. Regular "spectra" (IMO regular samson amsteel) progressively lengthens until it gets to its limit at which point it won't creep anymore. It isn't necessarily stretch because it doesn't spring back.
I had a set up on my 6.0 with amsteel blue as the whole bridle system and ended up actually splicing eyes in the center. I looped it once when it creeped out some. I use vectran on my blade and the knots tighten up a little resulting in loosening of the bridle line slightly but not as much as plain amsteel (spectra). My suggestion would be to continue tweeking the spectra you are using (tie the knot further up/down or whatever) until it gets to its "limit." Or you can switch to vectran (yale crytalline is covered vectran or you can use vectrus) but it doesn't handle abrasion as well as spectra. For what you are describing- the loop that is only 8 inches or so, small diameter crystalline would work perfect. You could also use amsteel blue or ultrex- they don't seem to creep as much as regular spectra.
hope this helps and doesn't confuse. probably more babbling than you were looking for <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" />
