I can only assume it's my monitor which enables this to work at 50hz with no stuttering.
When a 50hz screen mode is first initialised I get stutter as you say, but resyncing the monitor (a press of a button) then gives absolutely stutter free 50hz. No tearing, no stutter, no micro stutter. So as I said in the post I made earlier in this thread with my config link, it might not work with anyone else's setup.
Other users have reported stutter free configs, so Indivision is definitely capable of delivering good full screen 50hz results with a lot of effort and trial and error, just not with all the monitors out there.
Obviously this is a bit crap as IComp should not have left us with a product which is so difficult to configure and so flakey with different setups, but that's a different argument for a different thread. IComp appear to have decided that the config and firmware is 'good enough' and 'fit for purpose', which is clearly disappointing as so many users are obviously still struggling to get results they themselves deem 'fit for purpose'.