CS 727 Barometric Pressure Problem

Firslty apologies if I'm posting in the wrong forum.I'm experiencing lately strange behaviour in FS9 with Barometric Pressure with Captain Sim's 727 when running with real weather conditions.The only other addon I have is PMDG 737 which gives me no problems whatsoever.What's happening is...I'm sitting on the runway to takeoff in Captain Sims 727. Outside weather is Low pressure, rain and lightning.Altimeter is indicating -1000ft !! for the runway. Then it magically starts increasing till after a minute or so the altimeter is now reporting 10000+ ft !!??!!This is only occuring in this one addon ??I've reinstalled Fs9. Reinstalled FSUIPC (version 3.125)Now if I clear the weather and opt for clear skies, all works. :(Any thoughts ??As you can imagine it's somewhat impossible to fly with this continually changing!-Liam McMullan