Yeh, at first most of the strings did the slipping thing, but I stretched them (wobbled them left to right) as I tightened them to stop that from happening and that worked.

Having played for a little while, I am starting to see the problem. When I have the 3rd string in tune as an open note (I am using a digital tuner) it plays flat when I fret the string... like say I play a basic A-shape, the string is VERY flat and sounds awful. However, when I play a basic D-shape it is fine because the 3rd string is being played open and not fretted.

I started to use different amounts of pressure to hold down the string on the 2nd fret, and moved it back and forward within the fret space while holding the string down. The amount of variation in pitch is huge.... I'm starting to think that having a .20 unwound string as the 3rd just isn't going to work on this guitar... it's just too malleable/bendy and is too easily affected by the positioning of the finger when fretted.

Before I owned this strat guitar I had a epiphone gibson SG (which I still have), so restringing is very different for that one.. just finding so many little things that can go wrong with the restringing process and not finding many answers to them... the restringing guides on this site all assume that it's a perfect world and nothing goes wrong during the stringing process..

Should I just go back to the wound string? It's a .26 in this gauge set I am using (ie. 9, 11, 16, 26, 36, 46).. having .20 as the third doesn't seem that much of a difference.. but since it's unwound it appears to be making a huge impact.