I had this problem on another PVR and it turned out to be a faulty internal power supply. The regulated voltage to the tuner was drifting out of spec and causing the signal breakdown you have described. The symptoms were ambient temperature dependant, as the capacitors in the power supply were effected by temperature. It was worse in cool weather.
Possibly with two tuners drawing power it is worse. Just a long shot.
Possibly with two tuners drawing power it is worse. Just a long shot.