As for electrical, I would start with the ignition switch as this exact thing happened to me on my 284 the day it was delivered(they are not the highest quality). The contacts for the "on" position were bad. The tractor would start fine but after a bit all the gauges would be dead. Jiggled/Turned the key off then on and it would work again for a bit. Placed an electrical jumper(wire with alligator clips on each end) across the two switch conections that the "on" contacts would normally connect to continue with my breakin and prove the problem till the dealer could send me a new switch assembly. Received the new key switch in 2 days and all has been well since.
As for hydraulics, if that valve is similar to mine, it is a adjustable check valve. On the end of the shaft that the knob is attached to is a cone held in place with a pin so it can move up and down. The pressurized oil from the raise/lower/float valve that lifts the 3PH comes in through and goes out of the bottom of the hole that the knob is threaded into. It flows up past the cone and into the 3PH lift cylinder through a port in the back of the hole about half way up. When you screw this knob in, that cone on the end of the shaft hangs lower and lower getting closer to the inlet/outlet hole at the bottom(the bottom of the hole is tapered to match the cone). When you lift the 3PH, pressurized oil comes in from the bottom and pushes up and lift's the cone(which moves up and down freely on the end of the shaft) and flows by easilly to lift the 3PH cylinder. When done lifting, gravity pulls the cone tip back to it's lowest position. When you lower the 3PH, oil must flow back down around this cone which is lower into it's tapered hole and more restrictive to the returning flow so the 3PH lowers slower. The knob position controls the size of the gap that restricts the return flow which controls the lowering speed so you can keep a heavy impliment from slamming into the ground. IF you screw this valve in too far, you can actually hydraulically lock it in place with a impliment/load in the air. I have done this with a heavy brushhog attached and could not loosen that knob by hand(raise the cone against the hydraulic pressure from the impliment weight forcing it down). I had to put a jack under the impliment to get the load off the cylinder then the valve opened easilly with 2 fingers. There is a set screw on the front of the block that the knob/valve screws into. You want to make sure this is loose enough to rotate the knob easilly. Once you get the knob to the setting/lowering speed you like, you can keep it from moving/getting bumped with the setscrew. With the 3PH fully lowered(no load on it), you can remove the setscrew and unscrew the knob completly to see the cone assembly for yourself. If there is no load, no oil will come out. The threads on mine were dry and had some rust on them so removing the knob got some hydraulic oil up onto them to keep things moveing smoothly
On my 284, this valve also serves another purpose. I have a quick connect port under my seat that is desinged to feed a cylinder on a dump trailer. This quick connect is tapped into the system between the lift/lower/float control valve and that speed control knob. It sees the same pressure the 3PH cylinder does. By closing that knob down all the way, it prevents fluid from reaching the 3PH cylinder and sends all the pressure/flow to that quick connect(or back through the saftey valve to the resovoir).
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