Shift Kit

I think you are going to have to.
Probably a stuck valve or something like that.

Does it have engine braking in first?
(or when you let off the gas, does it act like it's in neutral?)

The valve body should be well flushed and all the valves you can see with the valve body split open should be moved back and forth in thier bores to flush out any contaminants.

It could also be the governor sticking, but lets get to that if the time comes.
The question in red is VERY important.