generally when you loose highspeed it is associated with safety switches like the pothole protection circuit that is used when the platform is elevated. I would check those first. There are switch(s) located in the pothole arm linkage and also at the base of the scissor arms that trigger when the platform is elevated to slow the lift travel down. If those check out ok then you will have to resort to checking hydraulic pressures down on the hydraulic control valve, one of those valves may be sticking. There are a couple valves that deal with fwd/rev travel, a dump valve and some other valves like a flow control valve and a check valve.
You can go to JLG's website and get the service book and it will guide you step by step through the process in the troubleshooting section. They are serial number specific so pay attention to that when looking it up.