I have an '87 ranger with a 2.9 liter v6. I've been having some troubles with the fluctuating idleing. When it is parked it idles high, but when it is on the road i can barely keep it running. So far i've replaced: plugs, wires, ignition coil, throttle positioning sensor, oxygen sensor, fuel and air filter, and the fuel pressure regulator

Does anyone have suggestions on what i can try next, anything will help?

 

Try the IAC sensor next, sound like a good candidate for replacement.