[ARSCLIST] Nakamichi tape deck problem

["RL C PayPal`" <sirnailheadjr@xxxxxxxx>]

Hey folks...I've got an old Nakamichi 480 2 head tape deck that has been
giving me some grief of late.  I've noticed that on some tapes I play the
sound will start out nice and clear and slowly get quieter and muddier
over the course of several minutes or more.  After getting several songs
into something I'll realize how awful it sounds and stop it, but as soon
as I hit play again it comes back in clear as a bell again, and then sort
of repeats.  

It doesn't seem to happen with all cassettes.  I'm spinning the Led Zep
box from Atlantic and it seems to be playing OK with very little loss of
sound.  I've even stopped it several times after a couple minutes just to
check and it's seems to be the same most of the time.  When I forgot and
left Led Zep playing for about 20 minutes before remembering to stop and
check, there was a bit of an improvement, after restarting, but it was
much less obvious than on most of the 6 or 8 things I've tested tonight. 
Some, but not all, of the tapes I noticed getting muddy on were comps
recorded on relatively cheapo Low Bias TDK/Maxell type tapes, while
another was a magazine freebie comp that was probably a C-60 or similar
quality.  The High Bias/Chrome/Metal tapes seem to play a bit better
without losing as much quality or as fast, but I've only checked a small
sample.  

I picked the deck up pretty cheap with the plan of transferring a bunch
of old tapes, of varying quality, of live gigs, old demos, obscuro bands
I was in, that sort of thing.  I've never had it serviced, but I did
clean off the heads and rollers when I got it and there was nothing
obvious to my eyes in the way of buildup or gunk that would be causing a
problem.  I've never heard of a problem like this and there is nothing I
can think of that would cause the sound to slowly get muddy then
instantly reset to clear sound when stopped and restarted.  Any ideas? 
I'm hoping to avoid laying out much to fix the problem, but if I have to
does anyone know of a good tech and/or Nakamichi shop in the Maryland
suburbs of DC?  Thanx.  Randy