Sounds to me like you've got a fuel problem. A leaky line, bad fitting, blocked clunk, dirty filter or insufficient tank vent maybe.
I'm not sure about your earlier problems, but I was at the AMT factory Friday a week ago (11/11). They were running the s**t out of your engine trying to make it fail. I heard it running over and over throughout the day. Full throttle, idle, quick spoolups, quick idle cuts...everything they could think of.
'Low RPM' error is what you get if the fuel flow quits or is significantly reduced. A sudden fuel stop (with subsequent sudden lubrication failure) is definitely BAD on the bearings, especially if you're still airborne and windmilling. Rapid shutdown from full throttle while flying can 'shock cool' the engine causing the thinner parts to cool faster than the turbine wheel. This can occasionally cause a temporary lockup situation. It will be OK after the temps equalize as long as you don't try to turn it. I've had this happen a couple of times on my jet sailplane when I shut down in flight after a hard acro routine (lots of full throttle time), and left the engines exposed to the cold air while landing. (BTW, it's still running.)
I've been running two AT-450's on my jet sailplane for a year now without a single maintenance issue, failed start or shutdown. The AT-450 uses most of the same parts as the AT-400. I'm running these engines HARD! 20 minutes or more at 106,000 RPM. I typically burn 8-11 GALLONS of fuel per flight. I have over 13 hours on one engine and 10 hours on the other with no signs of any problems. I've flown airshows at 106 degrees, and self-launched at a 10,000' density altitude (hot day at 6200' MSL). I've cruised at 100,000 RPM for almost an hour. I've restarted at over 10,000' In my experience, these engines are bullet-proof. If I didn't firmly believe this, I wouldn't be using them to fly my own butt inverted at 50'.
As for the warranty issue, the details of a business purchase can be very complicated. I seriously doubt if they are doing anything illegal. I got caught up in this myself with my spare engine, which had gone back to the old company for a 'checkup' before the airshow season started. I just got it back about a month ago. Throughout the whole ordeal, I found John and Norm to be very easy to deal with, and willing to work to a reasonable solution. I feel that AMT-USA is in very capable hands. Norm has a great deal of experience in handling large-scale production contracts, and is making sure things are done right before proceeding. There is a saying that goes something like "the only thing more over-rated than owning your own business, is natural childbirth". These guys have taken on a lot. I'm sure there will be some teething pains, but I am confident they can handle it. I look for AMT-USA to do great things in the future.
Bob Carlton
Silent Wings Airshows
www.silentwingsairshows.com