Ive replicated the behavior that youve reported. I see various spikes in the bit allocation on some frames. This issue has been reported in these forums before. Please see: http://software.intel.com/en-us/forums/showthread.php?t=79379
Unfortunately, the UMC codec does not have robust bit rate control features. They are intended to be used as examples and as such, they dont have the same support for BRC as a production codec would.
On another note, I checked the same source clip against the Intel Media SDK. While the Media SDK does not support MPEG4, it does h,264 and showed no spikes in bit allocation. Perhaps this is an alternative? You may find some possibilities to work around the issue as documented in the other thread as well.