In the duplicate thread ( http://www.rcuniverse.com/forum/fb.asp?m=10397133 ) you mentioned that the 75AX was problematic on a different model, possibly due to improper break-in; and possibly due to tank height. Before installing it in any other plane you need to bench run it to be sure it runs correctly with a proper tank configuration.

I'd try that before spending $100 to repair the 55 or $210 to buy a 65.