Ive flown in some heavey wx in SEA but Ive never had the LLZ bars fizzle out on the 737-300 TV screens yet. Normaly they wink at 1500 RA during the test but of course it is to say it cant happen. Probabley due to moisture entering the equipment bay somehow and affecting the SG componant of the flt management computer itself, or the internal self-test itself screwed up. Can happen when the rains heavey enough to register on the radio alts.