I had a problem like that with a 1840. With my problem it was the linkage for one of the control arms was wore a little. I fixed this my adjusting the linkage to the hydostaqctic which is right under the seat. The adjustment is attacted right to the small hydrostactic levers. So when it moves and you have to hold one or both of the control arms back or forward this is most likely your problem. Keeping that in mind you only have to adjust the side that to move the control arm to keep it from moving. If this doesn't help at all let me know. One of my good friends has a 1845c and I can ask him or someone else here can help you.