Video freezing in Recording View

Video all looks fine in Passthrough View (and when capturing with Media Express) but in Recording View is freezing every few seconds then speeding up to correct itself.
I've tried all sorts of tests, upgrading, reinstalling etc. but am still stuck.

I'm getting these messages in the terminal log by the way
```
[v210 @ 0x7fb1ef705100] Application has requested 33 threads. Using a thread count greater than 16 is not recommended.
[Parsed_amovie_0 @ 0x7fb1f01061c0] Channel layout is not set in output stream 1, guessed channel layout is '7.1'
```
Could be relevant?
Any suggestions?

Thanks!
Nigel