I think I would start with compression. Get a compression testor, not expensive and test the compression. Should be in the range of 100-150 per cylinder depending on wear. Anything less makes the motor suspect. If the motor comes up to specification for compression, then I am with the others, check to see that the distributor is set up correctly.
Trace your No. 1 wire to the distributor. Directly under the tower for No.1 wire, put a vertical chalk or paint pen line directly under the center of the No.1 tower. Remove all the plugs and put thumb over (not in, lol) the plug hole. Crank until you feel a puff of compression. Then look at the timing pointer and flywheel at the bellhousing. Rotate the motor clock wise by hand using the fan belt. Keep a very sharp eye for a mark on the flywheel that looks suspiciously like a little steel BB imbedded in the flywheel. Get the BB to line up with the pointer. Remove your distributor cap. the rotor should be pointing pretty close to, if not exactly on, the paint stripe you put on the distributor body.
If its not, loosen the screw on the distributor clamp, not the Hold down bolt, and turn the distributor so the rotor lines up with the chalk mark. Put it all back together and give it a go and see what it does..
