Thom,
It sounds like you might be running rich.  If I remember right, If your car 
runs better
cold than it does after it warms up, it could be a sign of a rich mixture. 
 A 28 PICT
is pretty old. Unless the jets have been replaced they are probably 
"wallowed"
out some.  Did you say you had black smoke?  On acceleration or 
deceleration?
Any backfire on deceleration?  If so either you have a rich mixture or an 
exhaust
leak.  If you want to re-jet your carb, Gene Berg still lists a pretty good 
selection
of main jets in his latest price list.

Charles Armstrong
'70 Bus

P.S.  When I bought my Bus, it had a hole in #3 piston(overheated and lost 
power
but the guy "couldn't see anything wrong" and drove another 50 miles on the
freeway).  It would start and I could drive for a couple of miles before 
power would drop off.  When I replaced the pistons and cylinders I did the 
quick rod bearing
test ala Muir.  No problem.  Unfortunately after increasing the compression
with the new p/c's, it was too much for the marginal main bearings(hurt from 
mileage
and overheating) to handle.  Kept getting intermittent engine seize. 
 Warmup=lockup.
Cool down and would run again. Only make it about a block.  Bottom line- 
depending
on the age and general shape of the engine if you are going to put new p/c's 
in
you may consider doing the bearings too.