PCB would be "printed circuit board" meaning the main circuit board of the
instrument cluster.  Some of the instruments like the speedo and tach
contain their own circuit boards.

Regarding the intermittent temp and fuel gages, I have found that just
removing those instruments from the main circuit board and cleaning the
connector contacts will usually fix them.  Each of the instruments plugs
into the main board and is held in place with one or two screws.  Some
people say that you should also resolder all the soldered connections on the
main board while you have it out or at least inspect it for any 'cold'
solder connections.
 If that doesn't fix it, the next step would be to buy a used cluster on
Ebay for about $75 and swap out the faulty gages.  Of course there's always
a chance that the gages in the used unit are faulty also so it's best to
swap the whole cluster first just to see what works.  If everything works
and you don't care about the mileage you can just leave it in.
As you can see from the above, when you get done with this you will be an
expert at removing the instrument cluster.  I could get mine out in less
than 15 minutes after the third time.

My E30's gauges stopped working; turned out some wires had come off the
back of the instrument cluster because the nuts holding on the ring lugs
had come unscrewed. You might not need a capacitor.

Ron