In my manual it makes the point that you can climb at M0.35 which you hit sometime after 15000 PAlt. So I bring the IAS down when it rises above M0.352, though at higher altitudes you get near the stall - when Fsx wind speed changes rapidly (eg a sudden headwind component to tailwind) this can happen.
Incidentally, in standard met conditions in temperate climes I can get to max altitude when fully laden, buit it takes time and distance (150 nm) - you need to settle at 15 or 16000 ft & see if it can maintain M0.45 (which I find is a realistic figure). This equates to 276 TAS @ std met conditions, (M=39 root Temp K degrees).
If your speed settles higher or rises @ 96.1% rpm & max save temp, go upt 2000 ft & see how that goes. you can use higher rpms of course, but being Scottish I believe in fuel efficiency!
In hotter/tropical conditions sometimes 16000 ft is the best you can get with those parameters. FS9 & FSX just make the temp @ alt 2 degs cooler per 1000 ft than the surface temp! This is not necassrily the case in the real world!
I forgot to say, 276 TAS/M0.45 is @ 20,000 in std conditions!