Oh my, I hope you don't go out and buy a new machine just to free motion on!!! You don't need to!! It really is all practice. You can even free motion on a TREADLE and people on this board do!
I agree with those who say it sounds like a tension issue. What thread are you using? I tend to use a REALLY high tension with cotton threads and a really low tension with polyester threads. Just do a LOT of playing. Seriously - get a test sandwich put together and start with your tension at 1. Go all the way up the dial slowly and look for improvement on the back AND front as you go. It's like playing "hot and cold". Once you find that magic spot you're good to go! You can pretty much count on being able to turn your tension to that number whenever you are using that particular combination of fabric/batt/thread/needle.
OH YES, what needle are you using too? THIS is huge! People don't often realize what a BIG difference needles make. If you're using 40wt or 50wt cotton you want something like a 90/12 sharp - if you're using a 30wt or 40wt poly you're going to want something like a 70/10 sharp. The difference between the two is when you're using the wrong needle you'll get skippet stitches and thread breaks.
Good luck!!!
