I climbed normally to the cruise level and the plane was flying fine when it gradually started losing its speed and was unable to hold at the altitude. even pushing the engines to max wasnt enough. I noticed the ambient temp was much below than it was a while before. I had to decend by about 10000 feet to regain speed or the plane would have stalled. This has happened twice so far.Is this a bug in fsx? how do I avoid it in my flights?

To forestall any tasteless comments about recent tragic aviation news versus the way that you fly, I will quickly point out that if the pitot heat switch is not turned on, the pitot tube outside of the aircraft ices up. The sensor in the tube gets fouled and you get incorrect flight information in your aircraft avionics, which can lead to mistakes by the autopilot and possible engine failiure. If you recover at a lower altitude, the air may be warm enough to thaw the pitot, if the heat switch is not on. Best to include the pitot switch position in your checklists! I am assuming that you are flying a jetliner with enough complexity to model pitot operation. Jeff ShylukSenior Staff ReviewerAVSIM

Icing doesnt seem to be a problem here as my ground speed too was lower than usual which indicates the plane was actualy flying slow. I dont know but can temp effect GS too?

Other than icing, I suppose the obvious questions are:1) Which aircraft?2) What altitude?3) At what weight?4) Was the fuel system operating properly?

The pitot senses changes in air pressure at altitude, and uses that to calculate airspeed. Those calculations are passed on to your avionics suite. Just like you wouldn't get an accurate reading on your car speedometer if it was encased in a block of ice, a frozen pitot will give you false readings as well. If you are relying on any of the velocity measuring gauges in the cockpit and your pitot is frozen, you will get incorrect readings. If you are getting ground velocity from some other source other than cockpit instrumentation (shift-Z gives you KIAS, or maybe you are looking at something like Flight Sim Commander), and if there's a discrepancy between what the program is telling you and the cockpit readouts, then that would be interesting. Jeff ShylukSenior Staff ReviewerAVSIM

if its a ptop plane then you need to adjust the mixture ( red levers ) , because as you climb the air gets thinner so you have less air in the mixture, engine looses power, RPM drops , you stall you crash, generally above 3000 ft or so you nedd to pull back ( or out ) the mixture lever, knob and watch for an increse in rpm, ans leave it there untill you descend wher you can put it back to normal. works a little differnt with turboprops, but they all still have the icing issue.