I dont know but... When i walk my fps goes up and down from like 100 to 10 and than to like 50 and then back to 110 ect.
And i have the latest java build

So maybe you guys can take a look at that one.

Specs:

RAM: 3GB
Processor: AMD Athlon(TM) 64 X2 Dual Core Processor 5000+
OS: Windows 7 Ultimate X32
Graphics card: Nvidia Geforce GT630