It could be alot of things.  Replace your cap, rotor, wires, plugs and then
see if it still happens.  If your mileage is still bad, check your ground
straps (one on the left head and one on the tip of the tranny).  If these are
corroded or missing, this could be the root of your problem (I had a similar
problem and it was a broken grounding strap).  If you still get bad mileage
try putting in a new Oxygen sensor.  They are only supposed to go for 45k
miles anyway and if they go south they can really trash your gas mileage.
Have fun restoring her and I have a bunch of good used parts so let me know if
you need anything.
Thanks
Ken Wilford
John 3:16