Ditto on wheels. Min wheel size for most grass fields is about 2.25"-2.5", IMHO. (I've seen grass fields that were like golf greens, or maybe fareways. That's different, small wheels work there. But most fields aren't that low cut). If your field is like my jungle/field gets in the spring, 2.75" or 3" is more like it.

Having the CG foward makes it worse as well, for obvious reaons. If it's wire gear, try bending it foward a little bit if you can. (if it's retracts, then that won't work, obviously).

Also make sure your wheels roll freely, and are aligned correctly. The more easily the wheels roll, the better.