1983 Datsun 280zx misfire!! PLEASE HELP!

Hey guys I am currently new to these forums, but I need some help with my Datsun 280zx, here's what's going on, one day I was driving down the road my car was running fine then all of a sudden it started running horrible like it was missing. Here's the things I've checked and done but still runs the same:
I've checked the spark plugs to see how it was running either rich or lean, it was normal. So I checked the spark plug wires with cylinder deactivation canceling each cylinder out at a time to see which wire was bad.. None were I even changed one of the spark plug wires still ran bad. So I checked & cleaned the EGR valve also changed the PCV valve.. Still ran bad, next up I thought it had to be a fuel problem so I checked all the injectors & they are operating properly. I haven't had a chance to check for fuel pressure but I just changed the regulator so I doubt it's that. It has a rough idle in park & in gear but seems to run fine until it shifts into 3rd gear then it runs horrible and doesn't want to accelerate but it still downshifts & has the same power it did before it was running like this. I have put fuel injector cleaner in so we will see if that makes a difference. I'm really hoping its not a compression issue if so looks like I'm gonna have a unpredicted rebuild at the worst possible time.
PLEASE HELP!! Thanks for any help or suggestions.

I feel your pain..I have a 1981 280zx turbo that does basically the same thing..I have replaced the mass air flow sensor , plugs , plug wires, fuel filter , had the Injectors cleaned and when I punch it , it just pops and sputters...if you find a solution , let me know and I will do the same...

check all connections including battery terminals and that everything is grounded well. These cars are getting very old and most of the issues I have ever seen is always a bad connection and or bad ground.