Does the timing become early or late? Does the dwell change?

If it's still the original points style dist. it may be time to pull
it and have it completely rebuilt.  The comment about lubing the
rubbing block was a good one though.  It's possible however that it is
binding up internally and retarding itself as you drive, and only a
rebuild can fix this.  Also the advance mechanism may be sticking
(either vacuum or centrifugal) basically you gotta take it out, pull
it apart, and look at it.

Personally, unless this is a 100% original car, I'd be tempted to
replace the distributor with an early 70's HEI unit just for no-
maintenance reliable sparkingness.

nate

Mark the distributors position and check the marks next time it's causing
you grief.

At least that way you'll know if it's physically moving or something
internal is messed up.

Also check:
1) hold down bolt is tight
2) the vacuum line for the advance. It has to be hooked to "throttle
vacuum"
3) side wear/play in the distributor hose where the inner shaft spins.
(should be damn near none)
4) timing chain back lash (8 to 10 degrees max)

When you "adjust the timing", is it actually changing from it's
original setting, or are you changing it to compensate for another
problem?!?!

You do not have to pull the distributor to check the mech/vac
advances. Pull the rotor off and check the mechanical weights and
springs. The weights and mechanism usually rust. WD 40 or CRC will
take care off that.  Vacuum pump for the vacuum advance.

If it's "binding" (not spinning free), and nothing is worn, you don't
have to rebuild it. You can pull the distributor, knock the roll pin
out of the drive gear and remove the gear and shim. Twist and pull the
inner shaft up through the points side of the outer housing. Sometimes
you need to take a can of carb cleaner and with the red hose spray it
into the shaft/hosing to cut all the built up oil. Clean the shaft on
a wire wheel and clean the hosing with the carb cleaner and a small,
round long handled wire brush. (usually found in the plumbing dept)

If it is a distributor problem, and it doesn't need to be "O" riganal,
then yes put an early HEI in it. Very easy change over! And much
better. Most off the time you don't even have to change heat settings
on the spark plugs. You will have to monitor them for awhile to see
for sure.

I've seen impala's of that vintage with bad motor mounts that would
allow the distributor to hit the firewall.  However, it would break the
cap, not change time.

For less then $100 you can buy a Petronix electronic ignition system
for it.  That will eliminate the points and  you should never have
that problem again.  Of course, if your distributor is rubbing against
the firewall then something is not right, perhaps it needs the engine
and transmission mounts replaced.