What pump for a pond where sediment keeps restricting flow?
Put the pump in a 5 gallon bucket with two bricks in it; something to keep it off the floor of even it. Improvise some kind of intake screen if necessary.

Install a 'T' fitting on the output, valved, so that you can bleed off the desired amount of over pressure in the line directly back into the pond. 

It's probably clogging where you have it choked down and possibly not making the pump happy from lack of cooling water flow.