Loads of the sanyo Microwaves I worked on had a steam problem with the touch membrane on the front panel...after a week or two of semi regular use, the thing would play up, the customer would unplug it and bring it in....by the time it got here it was dry and worked perfectly. Of course we sent them back out the first few times, putting it down to anomalies in their mains supplies etc. However, after a couple of them reported that the same problem returned, a service call was arranged instead, and one of our engineers caught it in the act on-site.
So if it starts playing up again, disconnect it, leave it 10 mins, then remove the cover and unplug the ribbon cable for the front panel, rebuild it and power it up again and leave it sitting on standby for a few days and see what happens...