Strange and intermittent navigation in 12.1.3 forms

We converted to 12.1.3 from 11.5.10 over the weekend. Some users are reporting strange form behaviour. eg a <Tab> usually advances one field to the right (proper tab order) but sometimes it moves one field to the left. Moving to the next record in a grid is usually done by use of the "down arrow" key - but sometimes it doesn't work. Mousers like to click onto the next field or record - but sometimes the only way to advance is using the <Tab> key. A date field won't allow a change - but wait 10 minutes and it will