Since I installed 5.4c, I have been having ntll.dll crashes very frequently in any airplane (default C172 and no weather to 737NGX with REX Wx engine) Before 5.4c, I was running 5.4b at 35% airline and 20% GA, and 100% UT2, and never had a crash. I did a bunch of trial and error, and finally narrowed it down to MyTraffic. Once I disabled it in the scenery library, I no longer had ntdll.dll crashes (even after slewing around the world at 40,000kts for hours). I even used settings such as 10% Airline and it still ntdll.dll crashes.
As an example, last weekend I did KPHX-KORD-KBOS in the NGX. While sitting at the gate, I CTD'd after 10 minutes (Airline at 40%, no GA). Reloaded, took off, and it ntdll.dll'd around 10,000 feet. Thankfully, I saved the flight and relaoded it during the climb and it crashed every single time at the same point (around 5 minutes in). I disabled Mytraffic and no crash. After disabling it, I was able to fly for the 6 hours real time with no crash.
Just reactivated MyTraffic around KPHX a few minutes ago, slewed from PHX to PAN (50nm northeast of PHX) and had the traffic bar open to see how many airplanes were up. I had a ntdll error immediately and only had ~15 airplanes on the traffic bar (only had it at 20% density).
I do have the saved flight -- would be more than happy to send it to you if it helps you out.
SLI GTX 580s