My 1995 Nissan D21 Pickup will not idle on its own. It seems to hold an idle at around 1200 rpm for 20 to 25 seconds then the tachometer will drop below 1000, sputter then cut off. Once it warms up it will idle, but it still does the same cycle. The only difference is that it drops to 1000 rpm, then returns to about 1200. It also has a bucking issue sometimes during acceleration, then when cruising at approximately 2000 rpm. Once it starts to buck, I have to let up on the gas, then slowly press the pedal to maintain a specific speed. No matter what gear I may be in. I've replaced the throttle position sensor, all of the vacuum lines, new plugs, rotor, distributor cap, valve cover gasket. Still no change in performance. Any advice?