2005 JLG 400S with rough idle

We have a newly acquired 2005 JLG 400S s/n- 0300084932 with a very rough idle. At start up the machine surges up and down from about 900 rpm's to 1200 rpm, then even when a function is applied and the rpm's increase the engine still surges up and down (rpms)

Any idea on how to fix this issue?

Thanks.

2005 JLG 400S with rough idle