I have a 1999 Nissan Maxima. The car started getting shaky when idle or parked, and jerky on acceleration. When in park, punching the gas stabilized the car for a little while. Shortly after this behavior, the Service Engine Soon light came on. It seems like turning the a/c off helped it run a bit better. Any ideas?

Check for codes. That should lead you in the right direction. Let us know what codes were thrown. Camshaft Position Sensors and Crankshaft Position Sensors are notorious for failing in Nissans.