problem started out as intermittent, now constant. codes for misfire 300-306 and 1399. runs very rough no power and barely idles without stalling. when this is happening the speedo also bounces around from 0-80 back to 60 all over. THE VERY Strange thing is if i plug my scan tool in the car runs PERFECTLY so there for i cant see any out of spec readings. completely stumped, anyone heard of such a thing. forgot to mention, all plugs, wires, cap & rotor have been replaced, egr valve and port cleaned. what i don't understand is how the scan tool corrects the problem!

 

Sounds like a bad ground. When the scan tool is plugged up, it is providing ground to the PCM.

Anyone been doing any work on the engine - around the TB area?

Anyway, I'd check that G101 (main ground for PCM) is clean and tight. Sorry this is the best pic I have of the location - #27 in pic.