My 94 F150 keeps surging. In nuetral it will steadily climb and back down 200 rpm. I really did everything I could think of. I changed the throttle position sensor. I changed the IAC valve. I unplugged the IAC and plugged the old one in with the new one on the throttle body dead. Same surge. I took the serpentine belt off and ran it to isolate anything run off the front, no change. I checked for vaccum leaks and could not find any. The truck keeps a steady 20 inhg. The vacuum does not change with the surge so I imagine that the fuel pressure regulator is not effected. I tried both fuel pumps also thinking maybe one was dropping pressure, same on both. I made sure the butterflies were not moving on the throttle body. I also blocked off the EGR valve to the manifold, same surge. I just finished a full ignition tuneup thinking that may be the cause. It was not a sudden change, just had been slowly getting worse and tends to do it worse when it's cold or before it is warmed up completely. I'm not really sure what to do next. I fells like I'm pulling sloshing water when I drive, so it must be dealt with.

I was thinking maybe the catalytic converters are plugged? Would a faulty O2 sensor be able to surge the engine like that? My worse case scenario is that something is wrong inside the trans like a pump giving up and it is pulling the engine down slightly. Any thoughts?