haha, that is funny.
I would just about bet it's your batteries. Our Yak's go straight up for as long as you want to. Oh, what prop are you using? If' it's the stock one that came with the plane, toss it in the trash can. They are as good as a Zinger. [X(] Use the ones that Jim listed a few posts back. If you can't get any, I'll sell you a few at Mac's. You're more than welcome to try out my batteries too if wanted to wait that long.
Here's the props:
http://www3.towerhobbies.com/cgi-bin...?&I=LXLKW5&P=7