Super strange issue.. stumbling under mild throttle

[fixkick]

Hi all!
Im newly registered however have spent about all of the past week scouring the forums for an issue similar to mine.. this is a long post and i apologize in advance, trying to be as thorough as possible. 

91’ samurai 1.3 tbi

The car will start and run and idle up then idle down like it should. Engine will free rev without issue and idle pretty well, altho could be a bit smoother.. Issue comes when i apply throttle and hold it steady. For example i apply throttle to rev the engine to 2,000 rpm and try to hold it steady, the rpm will surge up and down pretty uncontrollably and at some times will nearly cut out and die all together. Strangest part is the problem pretty much goes away when i restrict the return fuel line, idles near perfect and will hold steady rpms. Now i say thats strange considering ive read the fsm front to back 5 times atleast and done every single test and messed with every possibility ?? 

Things i have done 

New fuel pump fpr and filter, no clogged lines no kinks. Tested pressures and flow rates all is good. 
Ecm tested good and is controlling the ignition and fuel timing 
Tps is properly calibrated 
Every single input and output on the ecu tests within range (id assume all sensors are working as they should)
Checked all resistances and functions of all vsv egr valve bsbv and cold idle circuit 
checked atleast 3 times for vacuum leaks with carb cleaner propane and soapy water 
Gapped plugs 
Set timing 8 degrees 
Cleaned cap rotor and distributor
Not to mention a real tuneup when i purchased the car last summer 
Injector tests good and sprays reasonably well 

Im assuming its something stupid that im just over looking.. like mentioned when i up the pressure by restricting the return line i notice a slight change in the injector sound and slight change in pattern of the injector and the issue seems all but resolved. Injector is clean is not leaking tests within range.. not to mention the car ran perfectly not long before this issue developed. 

This issue i believe began about 2 weeks ago when i was driving home under normal conditions and the car cuts out for a second, comes back for maybe 5 seconds then dies completely (at this time i had just installed a new fuel pump tank sender and all that goodness) anyway.. i got out of the car kicked the fuel tank once and the fuel pump came back on. After that the pump died on and off a few times so i replaced it again with a better brand pump. ( wiring to pump bad)_

Now im stuck. Signs would point towards a dirty injector? It was cleaned not that long ago and still looks to be in great shape. ??‍♂️

Thank you in advance for any help its greatly appreciated!!

sammi, tbi, 1.3
wide open throttle tests do what? wot/flat ground or up hill. and parked,  all 3 good?
the ling block proves, fuel pressure is too low or injector is clogged, or ecu running the injector too lean due to wrong input to ECU (map is prime input)

if the fuel pressure shows in spec, at idle and wot (and does not change idle to wot then we  know the list  is less at 2 things, clogged injector (1/2 way clogged) or the ecu sees input lies (map)

the injector has 10micron screens (at its rim) and is 4 times small that main filter, so,  is  always a concern if tank had rust or gunk in it ,ever)

the ECU has many inputs.
do you have 19" of vacuum at idle nd steady needle>?
map hoses clean  and both end nipples must be cleaned, and not using wrong manifold nipples to map sure.
ECT reads 300 ohms hot,   UNPUGGED,  = 180F WATER TEMP BUT , this is not wrong, as more ohms makes more fuel rates (colder)
O2 sensor , run with it unplugged see if fueling goes better ,, richer.
DTC errors  does he ECU flash 12  (good)on demand?
IAT can not cause this, it is only a super fine trim.

the map reading to high for reality will do that.  for sure

https://fixkick.com/sensors/my-map.html

see this , if at cruise and 2.0v normal but yours is lower (wrong ) it will go lean.  best is test 3 or more data points on the map,  key on no start.. iidle and wot volts,

the hard part of all  is not easy top measure fuel pump pressure when driving,  what if bad wiring to it fails driving loose rusty connections, I used  remote gauge on mine to find this.

not sure me my 1.6L map above matches 1.3L  but will be close I think.
and 02 is not sick
all old sidekickes.. sammi, tbi, and trackers all have 02 sensors that have top much control so if bad , fuel mix goes straight to hell.  (1996 OBD2 cured that) then.

i feel your pain,  sure do and you will find it.
fuel pump wiring loves to fail, from left side harness all the why to the tank hot and ground both later big time fails.


good luck and happy holidays !