Just wondering what could cause it
Next time you see it take a photo of the radar screen including the settings.
Then send it to the radar manufacturer for their opinion. We did this with a 737 Bendix radar where there was a re-occuring problem with radar range effectiveness. Usual scenario was big CB's first appeared on the screen at 30 miles at high altitude despite the CB being visually seen at 100 miles and closer.
Despite frequent write up of the defect, it was always ground tested - serviceable. The manufacturer (Bendix) replied immediately, advising the problem was certainly water leaking into the radome and at high altitude the inside of the radome was coated with ice causing attenutation. On descent into warmer temperatures the ice melted and the attenuation disappeared. Hence the reason why the maintenance people were unable to fault the radar on the ground.
Following the Bendix letter, the maintenance people looked into it and found evidence of radome surface deterioration allowing moisture to enter. Problem was fixed and the radar was perfect after that.