Just wondering as there was a similar thread on AdWords a little while back.
It was noted that anything up to 10% of visitors will not make it to your site either because they click stop or back, or they are on dial up and the page is taking too long to load or because your server is serving the pages too slow and again, the user clicks away. Tracking software has also been known to slow down the page serving speed so it might be worth testing this on different browsers on PCs with cleared caches.
My spend on YSM! has reduced dramatically - it is now just plain annoying I spend a lot of time playing PPC cat and mouse on keyword phrases that these junk YSM! "search partners" are adding too quickly for YSM! to keep up with.