id make one  more suggestion to all of you... the 300  is an old  model . i have one  no problems  yet.  however  i have  theroughly  cleaned all pots  including the  model selector  and 5 way  toggle.

  i would   try  some contact  cleaner  on this  switch.    unfortunately   you cant  get at it  without    taking the  pick guard  off.   but  other than that  there  is  no disassembly.  if  you remove the  screws  and  lift the pick guard   you  should be  able to get at it  without  even removing  your  strings from   what  would  be the upper  side  if you  are  playing it.  use a  spray  nozel with the little  red  hose   give it a good  shot and  work  the  switch  while  wet. contacts  do oxidise   even  under the best of  conditions.

well im leaning toard the model switch   because    there all saying its   in  certain positions   on certain models  .  to me  and i could be  wrong    it indicates   a switch  either  the model  or the  5  way.     try to remember  are  all the  models that are being affected     in the same position  or  one next to each other  position on the 5  way toggle     or   is it  in different  spots.   if its  always say  position 4 /5   then  id clean the toggle    if its  in different places  i d  clean the  model selector.

if i were  doing  it  myself  id  pop that sucker  like a can of beans  and clean  every  switch.   but   im trying to be   specific     here. all switches  and pots   collect  dirt  dust  and corrode  its the nature  of the beast. even a regular guitar needs the pots   cleaned    over the years. a 300     dont  know  for sure  when they came out  but    you are  in the 6 to 10 year  range. unless you bought it  new  yourself  you dont  know   what conditions  it  has been  exposed to so    thats  the simplest thing to try  again  as to the   cable . i would  think all models in all positions  would be affected   were that the  case.

jmho good  luck