chad
Yep , GatCat is right , the front jack screw would be the first place that i'd look , if i remmember right there should be like .005 clearance between the barrel and the screw .
If that does not help the problem , you may need to pull the base off and lap it to the frame with some fine emroy cloth , and before you re-install the base make sure you clean all the old loc-tite out of the holes were the base screws down .
stimpy