I assume that motor is geared otherwise 9x6 is WAY too big a prop.

What is the "current draw of 2A" ? Speed 400s should be running at 8 to 10A. If it's only pulling 2A there's something very wrong.

Connectors ? If you've already tried 3 ESCs, 2 motors and a number of batteries there can't be many connectors that have been in there all the time so that doesn't sound like a very plausible explanation.

It sounds like you're changing too many things at once. From what you say you have changed everything in the system at least once. It's difficult to tell anything from that, sorry.

Steve