I have never used the silk covered thread. It should be fine because it's a natural fiber. I use the 100% egyptian cotton thread from Connecting Threads, just not the silk covered.
I think the silk portion is what is causing my problem. I got this from ConnectcingThreads.com too. Didn't notice about the silk but then wouldn't have thought it'd be "slippery" either! Oh, well, I'll just use it on things that don't need an exact straight seam.