Variable juddering video on external screen at 50Hz

I'm hoping someone can help me, loosing the plot a bit with this one now.

I work for a company who supply AV support for corporate conferences and the like. We use Macbook Pros to output video to vision mixers to supply screens and projectors.

We've been having an issue for ages that i'm struggling to track down. It started off with some older macbook pros (the last ones with the optical drives) but is also occurring with two brand new retina ones we've just purchased.

Basically if i play a video on an external display, the smoothness of the video isn't guaranteed. The amount of stuttering is constant throughout the playback of the video, but the amount of stuttering for each time of playing the same video seems to be random. Sometimes it's completely smooth, sometimes it looks like it's running about 5hz refresh rate, sometimes it's somewhere in between.

I've removed everything in the chain so it's just a laptop (doesn't seem to matter which one) and an HDMI cable from the HDMI out to a TV. I've also tried thunderbolt to DVI then DVI - HDMI to the TV.

This appears to be playback software (i've tried two professional video playback programmes and also VLC and Quicktime) and video file independent (tried various codecs and bitrates). The machines are perfectly capable of running the video files. They're quite happy running on the internal screen with the external display attached. In fact if you span the video window between the two displays it's smooth on one and choppy on the other.

Now after many days of trying everything i can think of it appears to go away if you set the external screen to 60Hz. Unfortunately we're in the UK where our native resolution for TV and video is 50Hz, so running at 60Hz gives you frame conversion glitches periodically.

Could this be something to do with having one monitor at 60 and one at 50. I'm assuming the two have to be synced to be able to move display objects from one display to another.

Running in clamshell mode with just the one TV connected didn't help.

Forcing the internal screen to 50hz (using switchresx) didn't help

I'd like to try a machine with 2 external 50z displays and no internal monitor but i don't have any machines that can do that.

Anyone got any thoughts, or anything else to try?

Thanks

Rob