Heat related LNBF frequency drifting is most likely the culprit. Try swapping out the LNBF if possible to trouble shoot. Try adjusting the LNBF LO frequency up or down in 1 MHz steps while observing a specific troublesome transponder frequency for quality level improvements.
A few years ago, I made several long distance trips to my mother's home to remedy a similar problem that caused the satellite signal to breakup between 4-7pm on very hot days. Servicing without a spectrum analyzer, I replaced the LNB on the first trip, The next trip the coax was replaced in case moisture had been wicked into an unseen damaged area. Third trip was with an analyzer, but unfortuantely for me (not my mother), the receiver was working when I arrived ..... AARRGGHH .... could only detect a slight RF pulse that I tracked to a nearby electrical transformer. Forth trip caught the picture breaking up and an incredibly strong RF signal from the power transformer. The power company crew found that the transformer was failing and the high demand during hot weather aggrivated the malfunction. The transformer was replaced and the satellite reception was not lost again until the BUD was disassembled some years later.....
Then there is the recent story about a self-install customer of an Adventistsat system... it would work fine if mounted temporarily on the ground, but poorly on the roof. After much trouble shooting with our telephone technicians, a local technician was scheduled and found that a local microwave tower's transmission was causing the interference. The neighbor's home was sheilding the dish when it was on the ground.
Many unrelated devices around our installs can be the cause of what originally be thought to be satellite equipment failure.
