From my little experience with FSX, I've already been having problems runnin it smoothly, and adding aircrafts and utilities like EZDok and sceneries. And honestly I feel that things shouldn't be so hard!

 

I was just wondering if I could get any recommendations for a much more stable flight simulator software.

 

At the same time, I'm curious to know what softwares does RL flight schools use for their simulators? And what softwares does the authorities use to do regular flightchecks with the pilots?

Each training school, home user etc will have different requirements... and for the real world stuff, the FAA or CASA or whatever body manages flight training and software approval have their own requirements for what software can be used to log hours for real world tickets etc. FSX is very resource hungry, but is the most "visually appealing" simulator available. X-Plane 10 is catching up slowly in this department. X-Plane can offer more realistic flight physics and more accurate performance however because of how it is designed/coded... FSX does have a lot more third party add-ons available which make it more appealing to many. There are other sim software out there like ASA On-Top and ELITE simulators too which cater more for the IFR/procedural training requirements (you don't see nice scenery out the window in these types of sims, because it is not needed for their intended purposes).

 

You could write many books on this one topic in fact, it is quite an open-ended subject, and each person will have different opinions too!