1995 Buick Riviera 3.8 L Super Charged Mileage: 149,000. My buick riviera has been idling rough when in drive. When I put it in park or on neutral it idles fine, even after a tune-up. Also when I get on the freeway when I hit speeds of 55 and higher the engine starts to shake. I took it to a mechanic he told me it was the transmission. I went to a transmission shop the guy said its not the transmisssion its the engine. Could you guys help me because nobody really seems to know what could be causing the shaking and rough idling.