I'd start by making sure your ignition and fuel systems are both operating the way theyre suppose to.
Start by checking for spark at all cylinders with a spark tester you get at any car parts store. Check the coil(s) and spark plug wires. Pull out your spark plugs, re-gap them, and torque them back in with a little bit of anti seize. Check mechanical timing and ignition timing because they are not the same.
Next check the injectors to verify they are working like theyre suppose to! It would'nt be a bad idea to check the fuel regulator, hoses, and injector seals. If you have an DVOM check the injectors' resistance, voltage drop, and if possible on a scope. Check your wiring from the ecu to the sensors/actuators as well because electrical problems are really common.
Check the MAF sensor for contamination and wiring. Go towards idle control devices like tps and test them according to Nissan's FSM. Check the engine coolant temp sensor. I dont know if the SR20 is the same as the CA18 swap as far as the throttle cables go but i had to get a new cable originally out of a sohc ka24de.
Aight good luck dude! Let us know if its still messed up.