my 98 was doing all the above, even the fame p0300. the p0300 ended up being the distributor, got and aluminum unit and it was a fix, then with the sluggish and poor acceleration it ended up being the rear cat. it affected even the shifting, under load it would throw the pcm off and it was hard shifting and then no overdrive. at first I even thought it was the egr considering that it had happen to me before and the egr crapped out, gave me p0300 and the truck was extremely sluggish and bogged out. I eliminated the rear cat, installed the two front o2s, put a straight pipe and that's all she wrote. power is back, trans is great thank god. another thing is that the clogged cat even affected me using the ac, everytime I would turn the ac on for the past 5 months, the truck was getting real lazy and at times would die while driving. I looked for vacuum leaks, I looked at the fuel pressure and no luck. before I checked the cat I looked at the cam retard considering that I just installed a new distributor but it was perfect. take a look at your exhaust flow and see if their is any restrictions even the slightest, I really hope I was able to help.

 

Motor ran smooth at idle and light driving around town. Could not accelerate on the highway. Replaced the CAT because it was rattling and I thought it finally clogged. Changed the plugs since they had been in for 100K. They looked great but definetly worn. Checked the splice pack on rear cross member. Checked crank sensor connection. All looked fine. I changed the gas filter that was also in for 100K and problem is gone.

 

Unplug the MAP sensor see if it still does it, if so, get a new one. An MAP sensor is to the left of the spider injector plug and has three wires.

 

If your advice is to the OP this was a 8 year old thread....lol I too have about posted replies to threads Jesus has started..they dont seem to be much traffic on this forum in certain areas ...But I actually had this same issue would only miss certain times could never figure it out....I built the motor myself so I was pretty familiar with the 4.3...turns out when I installed the distributor cap on one of the screws that came with it...I missed that it did not have any blue thread locker on it..And it was the one I put on the back by the firewall ...over time it worked loose and was causing a arc inside the cap..lol

 

My 1998 Jimmy 4x4 SLT was running rough when you pressed the pedal all the way to the floor board and it struggled to reach redline, but idles just fine. I went under the hood and unplugged the Mass Airflow Sensor and started her up and she revved all the way up to redline without any hesitation or problems. Your best bet is to unplug the MAF sensor and try it then, if she doesn't hesitate anymore, I'd recommend getting a whole new MAF sensor.

 

Mine was a clogged fuel filter