Hi again

Using Adobe Premiere Elements (latest version).

Shooting at 25 fps on a nikon d4, saving as HD1080p 25 video.
When reviewing the .mpg file on my iMac (mid 2011) there is the occasional sudden grey flicker, it doesn't go entirely grey just the frame appears dimmer for a split second.

Why is this happening? Also I use a glidecam, and when panning (not quickly) along it appears to jitter slightly making it hard for the eye to follow. I don't understand why this is happening as I haven't deleted frames?