Make sure the CG is right where it should be, not "pretty close". Make sure you've got the L.G on right, the wrong way will make it handle like crap-(technical term).
The wheel pants are no good if you're flying from grass. Do everything gently until it's in the sky.
Also, you may have a bit of a trim problem if it wants to sink back down once you've broken ground, try a little back trim on the elevator until you get it flying right.