Wow cross post or what - that had worried me all day why we were different - I decided following Mark's post to take a look at the relative timing on NENIN and NWDS with at least my twin input ability - the machine went bonkers unusable - fiddled for ages but even at X10 and unclipping the probes it was nuts... I noticed the pullup on NWDS I had tapped onto the lead of was a bit bent... went to straighten it and this happened (I wonder if it was already intermittent or a bit higher than rated):
I have put a new 4.7K on there at the moment and the single character corruption in CHARSET is gone - however I am missing the data on the very bottom lines (which I was never sure what it was but, was present on the SOC VDU anyway).
In graphics mode though non of the animated programs run correctly - the static images load and look fine. So I still have an issue.
I had wondered about different ram - I will have a hunt. I will also try a 2.4K pullup to be like yours.