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.

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1 answer 4 comments
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