Miva VA13 mini-excavator "bucking" problem when moving at anything but full speed

[raiken]

I just bought a new Miva VA13 mini-excavator at auction and everything works fine, except when driving forward or reverse at anything other than full speed, it "bucks" really badly on both tracks.

The bucket movement is smooth, there is no jumping motion.

 

[2manyrocks]

track tension???