Hi (for the first time)

I have a 1991 300ZX TT with some issues. Some weeks ago I noticed that the car ran very rough, from low to high revs. I changed sparks, checked compression and injectors and changed fuel filter. Voila! Ran great for 2 weeks. Until i stalled the engine and now Its back!

Whats wrong? The engine is hesitant and feels like it runs on 4-5 cylinders rather than 6. Electrical issue? Sparks and fuel filter are brand new!

Welcome!
A very likely source for your issue is corrosion at the various terminals associated with the engine management systems- in most likely order for you are injector connectors/harness continuity, CTS, PTU, and CAS connectors, and even the IACV connections and plumbing.
It's likely that your injector check helped connectivity before cold winter weather and humidity caused more corrosion and the return of the issue. You will want to inspect as many of the engine connectors you can (use of the FSM helps tremendously) and replacing any that are broken/cracked and of course too badly corroded. The new replacements you can get are weather-resistant unlike OEM so replacement actually is the best route to go and they are not overly expensive and can be a DIY project easily.
The FSM is also invaluable as you track further normal issues that will be occurring with your '91 due to the "old style" injectors and connectors, and the other issues above.
Good Luck!

DCaff300ZX - Thanks, but still guide me. What is a FSM ,CTS and CAS? I know the basics as i changed the plugs etc but not very familiar with electronics. Please guide me here.

What steps do i take next?

You are right, it´s cold here so you might be right.

Sorry with the acronyms...FSM is Factory Service Manual, CTS is Coolant Temp Sensor (top of coolant pipe/front of engine), CAS is Crank Angle Sensor (actually senses cam position for timing), and a few others that will come up in conversation and via the FSM's testing procedures. The FSM gives tests and flow charts to deal with specific problems, or point you in the right directions hunting down difficult problems. You can download that from the sticky on the front page I mentioned, or from NoLimit's sig, and elsewhere.
Oh, and it's imperative to be running OEM plugs for your specific (NA or TT) engine, all others an be a problem for specific small reasons.
Your main concern now would be finding and cleaning/replacing any connector that has green crud in it, or is broken or both. Check all harnesses for cracking/age and all vacuum lines for viability and potential vac leaks, and ensure your fuel system hoses and connections (and anything else really) are in spec and tight as well during the other procedures. As you go through the tests you will learn a lot about the engine and possibly/probably find other areas needing attention as well, given it's a 25-30 year old car depending on the year your car was built.
The FSM is invaluable to me for repairs and service but doesn't cover everything, so when you get going don't hesitate to ask questions here about details and about anything that doesn't make sense to you, or isn't covered in the FSM.
Now, time for you like the rest of us to learn a lot about a Z32...it's actually kind of fun while also frustrating due to the space issues and complexity of Nissan's little Baby...so Good Luck and have fun!

Spark plugs need to be specifically the NGK PFR----, if not that could also cause hesitation