Poor signal quality on Humax 9150T?

I have a 10 month old Humax PVR 9150T. Recently I have had problems with the signal quality; the picture breaks up and the signal quality meter shows variations from 0-100% and all points in between. At first I thought it was my aerial or electrical interference but all have been checked and no faults found. Then I noticed that it happened only after the PVR had been on, not standby, for 1.5 hours. Has anyone else had this problem?