Hi Patch. Not sure of the Model, is it an FOER 9/80.? If not, try to find out exactly what she is. I am sorry, I dont recognise FOR98.

Does it jump when just driving or when you operate the Hydraulics or the horn when just starting to drive.?

When it jumps, does it stop. Do you have to switch on again, or unplug the battery etc, or would it just carry on at high speed if you kept your foot down (you would need to be brave.!) let me know what you can and I will be able to help.
Regards. baz c.