I have a 1988 vw cabriolet that chugs intermitently. it happens more often when i let the gas tank get below half a tank. it sometimes chugs so badly that it feels you will get whiplash. it does not do this all the time. any ideas? it has been tuned up, had a new fuel pump a year ago, new fuel filter (there doesn’t appear to be any rust or stuff in the fuel filter).