I created a post about a month ago regarding a problem I was having wi the VNAV system and rate of ascent.

http://www.captainsim.org/yabb2/YaBB.pl?num=1323724979

I never could figure this out until I happened to notice that the model had no contrail at cruise altitude (34k - 37k feet).  I looked at the TAT reading, and sure enough, it read +55c (which is hotter than hell).  So, it made sense.  The VNAV was incorporating the TAT into the ascent rate and it adjust the cruise altitude accordingly.

I've been using FSInn to connect to VATSIM for online flights and I have the weather activated by default.  Doing this causes FSX to switch the weather option to User Defined.

I made a few test flights without connecting to VATSIM or using the FSInn add on.  Sure enough, the model operated like it should have.  I then turned off the weather function in FSInn and connected to VATSIM using the Real World Weather option in FSX.  The model operated like it should.  I had a nice 2,000+ FPM climb to 34,000 feet and the TAT read about -22c, which is much more reasonable.

So, if you are using FSInn (a great add on that's much more user friendly than Squwak Box) to connect to a virtual ATC world, make sure you disable the weather function.

Yup, it's a major and known issue. Happens with many add-on aircrafts and FSInn wx.