Humax PVR changing channels un-prompted?

We have been using our Humax PVR virtually trouble free for 2+ years but this evening, whilst watching BBC2 through the Humax, it suddenly changed to BBC1. There was no inadvertant contact with the remote control, it happened 'unprovoked'! I changed channels back to BBC2 and continued to watch the prog for a further couple of minutes before it again reverted to BBC1. This happened several times, sometimes with only seconds between swapping back to BBC2. I managed to get onto the correct channel again and quickly pressed record before it swapped back to BBC1 - it did however record the correct station. I tried re-searching and storing the channels but this didn't fix it. However, I became aware of a message appearing very briefly in the top right hand corner of the picture which said something like 'please wait' but it was only there (frequently) for ~half a second a time. Any ideas anyone??