It's not likely the whole distributor needs to be replaced. Inside it is a component, the ignition control module (ICM), that could be a problem; that part is only 1/4 to 1/3 the cost of the whole distributor. The ignition coil is another possible cause, but so are several other things; like wiring and connectors and the ignition switch. Is this an automatic transmission, are the problems with the tach and the "chugging" constant, or only once-in-awhile? Do the problems seem related to any particular engine speed, or car speed, uphill vs. downhill, hot vs. cold engine? Observation and some step-by-step testing are the keys to identifying the root cause, instead of just replacing parts; although those steps can be difficult for any shop if a problem is intermittent. Did both shops do a road-test, did they spend some time under the hood with instruments? Most likely the problem is with either the coil or ICM, but not both. My guess is the ICM, as it gets hotter and has more complex circuitry... but that's just a guess!