check the timing...it may be late...or you have a cylinder not firing?....check with a vacuum gauge....reading should be absolutely steady at whatever vacuum....

Check for the dead cylinder...just short them out one at a time with a test light and listen for the rpm drop...the one (or more) that rpm doesn't drop will be the ones to look at ....

hook the test light to a ground and puncture the terminal at the distributor (dis) tower/post and walla....