I had the same problem with my '70E and found that it was due to a poor ground. The ground connection to each of the instruments was a series of ring lugs on a black wire under the dash that were under one of the knurled nuts that holds the bracket mounting each gauge to the dash. This is not a real solid ground, as it turns out. Try a simple test: take an alligator clip jumper and clip one end to a known good chassis ground then reach under the dash and touch the other end to the metal shell of one of instruments while someone is watching the lights in that gauge. You may be surprised.

It is true that very old bulbs will build up some film on the inside of the glass from the filamenet cooking off over time, but it doesn't dim the bulb as much as you (and I) have seen.

Dave