So, I replaced the original Holly with an Eldebrock 1406 and a new filter. Car ran great the first 4 or 5 times out driving. Then the car would start to gallop slightly or have fast flutters of hesitation at cruising speed, regardless of what gear I was using or speed. It doesn't happen on the highway because the car is under load maintaining 75 mph? Also, I could be in third gear at 40 mph, and the engine is galloping until I start up a slight hill, putting the car under load again. Starting, maintaining an idle, and accelerating are all fine. Timing is set at 13 BTC; the carb was adjusted at install using a vacuum gauge to set the idle screws; all emissions hooked to the carb have been deleted/ plugged; vacuum advance is on the correct nipple with a new hose... I replaced the original 1985 mechanical pump with an Edelbrock mechanical pump specific to the new carb. No change in the problem. Not sure what to do next