Rough idle and stalling

When I cold start my engine, the idle is rough and uneven. It revs up and down to almost stalling (which it does sometimes). When I try to shift into reverse or drive while the engine is cold, it stalls. Also tries to stall when I stop at lights and when turning hard to the left. Replaced distributor cap and rotor, checked plugs, added STP fuel system cleaner. Still idling rough and stalling. Hearing a clunk when shifting. Possibly bad u-joint? Engine rattling like a diesel truck or pressure cooker in low gears, especially while climbing.