check engine light comes on and engine loses power

I have a 1994 F150 etra cab 6cyl with 138K miles. when cold the check engine light stays on and the motor won't start untill it goes out or I turn off the ignition and pump the gas pedal then turn on the ignition. It will start but runs missing and has no power, the transmission won't shift automatically, the exhaust smells like sulphur and gray black smoke comes out. The overdrive switch won't light up and the usual electrical static and engine noise on the radio (AM BAND)goes away and sometimes the radio display won't come on. After a while the light will go out and the engine runs smooth the trans works great and the noise on AM radio comes back, Or I can after a while turn off the switch and usually the light will go out and all runs great. For several hours all will run great, then it will light up again and start missing I have taken it to several shops, but none agree on what the problem is, because they can't read the codes on the computer. When the chk eng light comes on and all the problems occur the diagnostic code reader or analiser they connect to the engine won't work and won't reconize it. How ever once the light goes out the analiser works fine but reports that all codes are cleared and they can't make a diagnostic.Help!!!!!!!!! If anyone knows what the problem can be please post or send email to: ED at snipped-for-privacy@netzero.net or snipped-for-privacy@yahoo.com