Having some major running issues with my 2000 Blazer 4x4 (ZR2 package 230000 miles).

The vehicle runs ok in the rev range idle to about 1900. After that it starts to splutter a bit and the tach has a tendency to jump around by up to 500 RPM. The more you open the throttle the worse it gets especially up hills, when it gets to the point that it sounds like its running on no more than three cylinders and sets the 0300 code. It has once set the MAP low input code.
Acceleration is abysmal and non existent at this point. Also if the large breather tube from the LH rocker cover is connected, at certain points the tach again jumps, BUT, it feels like the tranny is downshifting then upshifting again. If this breather is not connected the tranny behaves quite normally.
I have dredged the net and have come up with so many conflicting opinions that its ridiculous. Someone even said that the timing marks on the crank balancer are pretty much irrelevant. Ive seen everything from vacuum leaks to a bad CKP.

Just for info in case it helps, my actron scanner says that the MAP is round the 11 mark and the IGN ADV is 18 deg at idle, and if you rev the engine this goes to 30+. Of course there is no way to alter the distributor with it being fixed.

Any help would be appreciated.

 

first thing I would do is fuel leakdown test