I had a very similar problem.... (OK identical problem)

You say you changed the fuel tubing when you were looking for air leaks... i think that is the problem. The MGT comes with a larger bore tubing than "the regular stuff". I swapped to some smaller bore fuel tubing before i realised this and the engine ran aufully. Replace the original fuel tubing, or get some of the "big bore" stuff (the big bore tubing is slightly bigger than stock so you will need to use fuel line clips to hold the tubing onto the fuel nipples. It will be difficult getting the larger bore tubing into the pipe (as it doesnt have a nipple) but it will go.

It is something to do with fuel flow and backpressure. Dont ask me why, but it is. Switched back to larger bore tubing and the engine runs perfectly.