Do a few test for me.

1. Check your battery to make sure that its balanced and charged at 4.2v per cell 8.4 total.
2. Check your blades, test, boil, blance and track.
3. Check your gearmesh.

Cheap Battery Test and Fix
http://www.rcuniverse.com/forum/m_6970199/tm.htm

Warped blade test
http://youtube.com/watch?v=_7PwNTXOWMs

Blade boil fix
http://youtube.com/watch?v=RmNv8OlG6GQ

Gear Mesh Guide
http://www.rcheli411.com/pages/howto.php?howto=7&page=2


If thats not your problem then test your 3-1 or 4-1, and replace the bearings. If you still have issues then try new motors. Please keep us posted.
Heres more for you.

Tips/Repair/Guides/Mods Links
http://www.rcuniverse.com/forum/m_68...tm.htm#6821632