a couple of things then you might look at, ESC temps, or if there are any connectors that might be loose when you are going straight up that might not be making a good connection. Other than that I don't really have any ideas, hopefully you find what's doing it though.

one other thing I just looked up is that your ESC is rated at 35A and the motor is rated at 38A max cont draw, so it's probably that when you are going up you are pulling the full juice that the ESC can handle, plus a little more, so the ESC goes into current limiting mode because it'll get too hot and self destruct otherwise. You may just need a bigger ESC. A wattmeter would tell you that for sure. You might either buy one, or see if you can borrow one at the field

edit:
oh here is the link I was lookin at
http://www.3dhobbyshop.com/detail.aspx?ID=1627