Engine hestitation and stalling

[NAsubielife]

My impreza suddenly began to hestitate a few weeks ago, like over the night after running perfectly fine for 6 months.

It doesn’t hestitate on full throttle, only when idling and all the way up to about 90% throttle

(My impreza is the N/A 2.0 GX btw)

I first thought of checking the spark plugs as i haven’t changed those in god knows how long ago… it was then i noticed an oil leak from the valve cover for cylinder 1 and 3, and saw that the spark plugs and boots were covered in oil and thought i found the problem, so i replaced the valve cover gasket and installed new spark plugs.  (obviously cleaned the boots, holes and valve cover for oil before assembling)

i had no luck in that and realized i had to check for other things. I checked the plug wires and they were all good, same difference in the engine’s rpm when pulling off 1 wire after another. 

I have also replaced the fuel filter thinking that it may have been very clogged, checked the air filter and that’s also in good shape. I also checked the maf sensor for damages and cleaned it. 
Oh and, i checked for any air leaks in the tubes leading to the airfilter box and found NO leaks. 
 

Important note: i have a CEL which shows cylinder misfire, but sometimes it varies which cylinder and how many cylinders it misfires on. I also had a code for lean fuel mixture, but that code suddenly disappeared the next time i checked. 

I’m getting really frustrated about this, but i am currently waiting for a new front o2 sensor that i ordered a few days ago to see if replacing that can help. 
 

Please leave any tips if you have!