LMR 400 has a Cu coated Al center conductor. This can, and does, have corrosion issues, particularly when the Cu is "nicked" when stripping the dielectric from it. With repeated heating and cooling will get water into the cable, it won't matter if it is upside down or right side up if it is not sealed hermetically. The antenna feed can also wick water into the cable in a 239/259 combo as the SO 239 is NOT hermetic.
That all said, if the cable has been up for a number of years and has been problem free, next time you have an issue break out a spectrum analyzer and take a look at what else is going on in the neighborhood. More often than not it is some moron's GD garage door opener somehow emitting at 1/3 of the spec'd frequency.... 433/3 = 144.3 MHz and if the LO is, like usual, running at 1/5 or 1/3 of the spec'd frequency it will be a noise source. LED drivers are also notorious for generating noise at obscene frequencies (don't ask me how I know this) as are fluorescent lamp electronic ballasts. In fact the FL ballasts can be absolutely impossible to filter/choke/strangle etc. as the RF is in the discharge plasma!
If a spectrum analyzer is too expensive (they make a cheap one for $50 these days!) tune an all mode rig slowly through the spectrum on cw and note the problem frequencies then correlate them with repeaters etc. and any unknowns. Make an effort to demodulate (AM/SSB/CW/FM/PSK/DMR/D-STAR/SYSTEM FUSION etc.) and locate. If there are any "mysteries" left after that run them down and localize the culprit.
Pretty sure it is your buddy down the block but I'd not convict him without real evidence....
Grover