OK, fitted an SSD in the last couple of months and apart from my mac mini server getting a bit hot (strangely as they surely run cooler than HDD), I have also been having trouble with my mouse losing connectivity. Recently it has got much, much worse.
I've read it could be to do with dust on the motherboard, so I dusted it down. Also, I have heard it could be interference, so I carefully cut a couple of pieces of aluminium duct tape (left the backing on) and covered the drive over. No change.
Read a lot about battery voltage in rechargeables (1.2v rather than 1.5v). Though it could be that.
Then battery size - some are slightly less tall. Thought it could be that.
Making it a favourite in the preferences was something else I tried, but that was not the answer.
Finally, stuck a bit of cardboard between the batteries and the cover and bingo. That seems to be the answer.
Guess the weight of the batteries must just be enough to make the connection flakey.