Air carrier pilot reported problems with the B737-800 weather radar not working correctly on numerous aircraft.

Narrative

I have written up four -800s in the last year for weather radar that does not accurately depict thunderstorms on the 160-mile scale. In visual conditions it has been obvious that weather was within and above our altitude (confirmed with WSI EFB radar display). The radars displayed no weather returns at all. When within 80 miles; the radar depicted some returns but all were hashed out in green; meaning our altitude was well above. Visual verification proved this to be false. Once within the 40-mile scale; it would more accurately depict intensity and height; but by then it could be too late to make a preemptive turn.Two Mechanics informed me that there was a known corruption of the data on the -800's memory that depicts return tops. The most recent Mechanic also said how temperature (heat) sensitive the transmitters are and he blew out a canister filter that was full of debris. We have recently been reminded of turbulence injuries to Flight Attendants. The scenario I have written about happening in IMC or night conditions; when 100% of our faith relies on accurate radar returns; could lead to inadvertent penetration of thunderstorms.