So I also have become tired of these issues. But I have so many addons, that I really couldn't say if it is really the PMDG 777.
So I decided to go through the painful issue of reinstalling FSX. I have an iMac using Bootcamp and took the opportunity to do a format and clean installation of Windows 7.
I installed FSX, and service packs. I then installed my various addons, REX, Orbx, Active Sky. Then I installed the PMDG 737NGX. I spent a week doing various flights on NGX, and had no issues what so ever.
I then installed the PMDG 777. I did a short flight from Brisbane to Sydney, and quick turnaround to head back to Brisbane. About 30 minutes into the flight spinning blue wheel started and FSX crashed.
It clearly has something to do with the PMDG 777 (NGX, J4100, Coolsky DC9, A2A C172 have no issues). I'm not saying it is not an interaction with some other add-on, but given it only happens with 777, means something isn't right.
Hopefully SP1 fixes this.
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Ryan P