When "it stops"...is that the ram that stops or is it the engine that is bogging down??
If engine is bogging down, other posters are right, you need more power at higher rpm's...
If engine just keep running and ram stops, flow is bypassing some where, most likely through the relief valve.....
If this problem occurred suddenly, it is very possible there is some debris stuck in the relief valve that keeps it partly open, that explains the fact that it split when rpm's (flow) is increased....when increasing flow through that little opening in the relief valve, the pressure will increase enough to move the ram....
Do not keep running splitter like this, there will be over heating problems.....
2c