PCC, the 2nd WAN traffic rate jump down to 0 sometimes ?

big trouble with PCC, could anyone help me ?
two WAN interfaces, using PCC , but the second WAN traffic rate jump down to 0 sometimes.
have changed to x86 and RB hardware, but the same problem is there.
WHAT IS POSSIBLE REASON ?

I have exported the configration .
export20120116.rsc (90.7 KB)