Erratic behaviour of block insertion point

I’ve been using a 3 pixel block insertion point for a few years now without any problem but in the last few days this has been acting erratically. It works fine for so long then the block insertion point disappears and there is no regular cursor either. After that it may act in one of two ways. Either a) clicking the left mouse brings it back or b) sometimes if I don’t notice it disappeared the cursor position can move backwards through my document, settle somewhere apparently randomly I am not aware of and then what I type overwrites what is already there - Ctrl-z puts it back of course but it is getting to be really annoying. Any clues anyone?