Conducting a flight from Singapore to Perth, PMDG Boeing 747.Took off, everything normal then started experiencing strange things half hour out.Aircraft couldnt climb any higher than 29000ft, was starting to stall, i had to descend to 25000ft.Thought that Pitot tubes may have iced up giving me false IAS and altitude readings, nope.It had me stumped.Declared an emergency, headed for WIPP, was descending normally at 8000ft where agl is 36ft, when i looked outside AND THERE WAS TERRA FIRMA ONLY JUST BELOW ME!!!!!!Slammed the throttles to the limit, pulled up, steadied the big girl then said to myself, #####!!!I found the problem, but can you guess as to what the problem was?I will tell you in a couple of hours.

Air Brake, landing gear, or flaps?

Pilot error?:)

Nope, had a QNH at WIPP of 756, you wouldnt get that in the middle of a Hurricane/Cyclone!!! :( Made a difference of nearly 8000ft.METAR string was 31/12/2009, 0600.Andy