My apologies if I did not understand the question correctly LatPilot. What you were describing just looked to me like a commonly experienced antenna tilt issue. With that theory eliminated I can only speculate as to what's happening with your radar that would cause it to behave as you describe. The best advice I can give is to have mx investigate.
One theory which has played a part in similar problems:
If this happens while descending into warm moist air I might suspect condensation as a possible culprit. Cold soaked radomes, antennas and wave guides can collect enough moisture to cause radar problems under some circumstances. Additionally older radomes sometimes entrain moisture in voids created by material delamination. Continued temperature cycles usually make the problem worsen with time as entrained water freezes and furthers the delamination.
Or it could be something else entirely!