I have a 89 nissan 4x4

check the choke control

try replacing your PVC valve or EGR valve also check for vaccumm leaks

Maybe catalytic converter

MyNiss. 88 fwd tk.was doing than after, tuneup.Ihad crossed vaccum lines on air breather.

check fuel pump pressure- fuel filter --air filter --egr valve if carboned up clean or replace and cat converter feel the exhaust if feels restricted may be clogging up

try idle air control valve

If it is bad gas you would be able to smell it usually. When gas goes bad it turns to like varnish and the smell ( believe me- once you have smelled it) you will never forget what it smells like. If the engine light is on, check the trouble codes. On a 1995 Nissan Sentra it was the first year of OBD 2 systems so it should have a hookup for the scan tool but if you don't have a scan tool you can still retrieve the codes from the ECU or the check engine light.
Under the dash behind the center console is the ECU. You will need to get to it(Don't disconnect it from it's wiring harness ) Just be able to access it and I believe on the side of the ECU isa Diagnostic Mode Selector which you will move to the left and turn ignition switch to on position(not start) and watch how the check engine light flashes. Sometimes it will flash fast and sometimes slow. Then it will repeat. Example a long beep then 4 quick beeps would be a code 14. Write down what yours is and get back with me. Good luck.

it could be a bad belt or signs that the fuel pump is getting weak put some STP gas treatment in the gas tank make sure you have at least a half a tank of gas before you do this ask yourself when was the last time you changed the fuel filter

sometimes the gasket under the trottle body goes bad. spray carb cleaner at the base of the trottle (not down into it) body while the engine is running. if the idle changes your on the right track. its an easy gasket to change.

Try clamping the fuel return line with the appropriate tool that wont damage line and see if it improves if so you need a fuel pump or pressure relief valve.I know you said you had fuel pressure but you took that reading while vehicle was idleing so you cant check for pressure in above idle conditions.Is your air cleaner assembly as original with no holes or leaks from air box to throttle body?Its more likely to be a mass air flow meter.This problem occured to my vette while i was on holiday there.cost$89 exchange.good luck