Hitachi zx 350 with lift problems

We just bought a zx 350 and it has about 5200 hrs. When it gets warmed up the lift control gets very jumpy, like the valve is sticking or something. Once the valve opens it's smooth but when you first pull back on the lever slowly it won't lift and you keep slowly pulling back on the lever then all of the sudden the bucket will jump up about six inches. It does this every time you make a cycle or raise the boom then lower it. Any ideas would be greatly appreciated.

Thanks