Jlg 800a won’t engage transport higher rpm.

Jlg 800a intermittently won’t engage engine to higher rpm for transporting or driving this boom lift. It does have a etc control that activates an actuator that engages to receive higher engine rpm. Is there a proximity switch that could cause this malfunction? It’s almost as the actuator is engaging but can’t, sometimes even to the point of stalling the engine. If I start it in the morning it will work for a few minutes.