IMX219 - inconsistent frame rate

Hi,
I'm working on IM219,
- ran gstreamer pipeline with format = NV12, width = 640, height = 480 and frame rate to 120.
- But getting inconsistent fps. FPS drops from 120 to 90 to sometimes 60 and then 40's and bounce ack to 80's...
- Do any one have any idea, hat might be wrong?

Thanks,