Had same thing in December, easy way to check is start her up then remove the coil on the the cylinders one by one, so you pop the coil plug out, if the idle gets worse reconnect and move to the next one, repeat till you find the one that makes no difference. I have 5 spares in the cupboard because I replaced all of mine, plugs too. I have all the part numbers if you need.
It just happens, mine was after Burgy got my one running again with the charger on it, happened under heavy acceleration, thought I'd dropped a cylinder the vibration was so bad.
Will be a coil or a spark plug.