Dangerous Gear Malfunction.

It should freewheel when you coast. Freewheel cogsets with damaged ratchet mechanisms may do this. Have your LBS inspect it coz maybe the whole hub needs to be replaced.

Quoted post said:

Originally posted by oil27
My MTB suddenly began to forward peddle by itself while I coasted. It was violent and wobbled me around. Fortunately I was on a back street. This happened several times since. Sporadicly it just catches. I am sure there is a fault inside the back wheel axel inside the the cogged part, in the gear really.

Can it be fixed or should I just by a new bike for this one was really very cheap? How can I fix it?

If it's a cheap one, it has a freewheel. The freewheel has a ratchet and pawl. If that mechanism is faulty, a new freewheel is in order.
Some cheap bikes have a freehub system. In that case, replacement of the hub is in order.... but it is likely that a whole new rear wheel could be cheaper... saving the old cassette.