99 runs fine until closed loop...broken wire??
 

Just recently got the blazer out from retirement, no license for a couple years, and it started running rough. At first it was just the occasional miss/stutter while cruising and then it gradually got worse to more of a miss/bucking over the course of a month. For the first few minutes it runs fine and then if you ring the rpms around 2500 it will start missing and puffing black smoke and you can watch the tach bounce up and down from the miss. I do believe it is losing spark for split second. It has never thrown a cel and I have had it plugged in and has no past stored codes either.
The fuel pump has been replaced and has great fuel pressure, replaced this when first started driving it again before it started acting up with this problem, about two months before it acted up.
After the problem started I have replaced all three oxygen sensors, map sensor, ecu, coil, cap and rotor, ign module, starter and battery with all new parts.
Also have cleaned the egr and throttle body. Swapped the mass air from a running vehicle and also changed the spider assembly and pressure regulator.
I put on a new crank position sensor and then it ran good(maybe a slight hesitation every now and then) for a few weeks and now back to missing/bucking blowing black smoke. It is not really drivable like this because you really need to be revving good to make it less noticable and it will kill when at lower speed/rpms and then you have to hold pedal to the floor to get it to restart.
So I guess the last lil paragraph is my question, I just installed another new crank sensor and have got it to operating temp three times but it still has the problem. Any and all help would be appreciated. I am in a wheelchair so if you need to know and specifics under the hood I will get a hold of someone to come have a look. Sorry for the long read and thanks for your time, Mike

did you happen to change the plugs and wires too? i see you had the cap, rotor, coil, ign module changed, but not the plugs and wires. i just want to make sure everyone is on the same page.

No the plugs and wires have not been changed, i'm sure the plugs will need to be changed after all the loading up its been doing. It runs like a boss until it gets to operating temp or closed loop, it's great the first few minutes of driving it so they wouldn't have anything to do with the problem.