The engine will not rev to more than 4000 rpm and at that rpm it shudders. on 2003 Ford Taurus

I first noticed that when using passing gear it would jump in and out of passing gear. I then noticed that in park or neutral I could not get the engine to rev to more than 4000 rpm. I have changed the spark plugs and the fuel filter to no effect. I have no idea where to go from here. I have been told that the rev limiter might be bad, but I can't find any info on that, and I am more inclined to think it might be the PCM? I had the codes checked and it is not showing any. The check engine light is not on. Any help would be greatly appreciated.

Asked by for the 2003 Ford Taurus
have a mech drive car with a scanner attached so he can read live data and diag problem.also have him scan poss trans codes
Ok I took it to the shop and had a mechanic drive it and it was showing no trouble codes. He said I needed a complete tune-up and to change the fuel filter. I have already do that?

Any more thoughts please?
what did live data show if there were not codes
I think he only had a code reader with him and not a scanner :(
try another mech the info would be priceless
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