Clutch chatter in my 2000 Ford Mustang 3.8l V-6

My newly purchased 2000 Ford Mustang convertible 3.8l V-6 has 162,000miles and clutch chatter/ car shakes after it has been driven for about 10-15 min. The car shakes only when I start in 1st or reverse gear. The previous owner said he had installed a “new” aftermarket clutch just before selling it last June. For its high miles the engine is awesome but don’t like the chatter. The clutch engages at about 3/4 of the way from the floor. I don’t notice any slippage. Any suggestions to fix the problem. Thanks, Jim