97 4.0 litre Aerostar acts like rev limiter at 2200 RPM.

My brother's 1997 Aerostar AWD 4.0 liter has suddenly developed a problem. At 2500 rpm the engine acts like it has hit the rev limiter.
The check engine light does not come on, there are no codes stored, and the misfire flag does not respond. If pushed continuously through the"cut" it eventually throws a code, (p230, IIRC) which is an indication that 2 or more successive PIP signals are invalid. This can be caused by shorted secondary ignition - but that would also trip a misfire code. There are no new PCMs for this van in North America - and installing a used PCM made no difference, Truck has new plugs and wires. Crank sensor checks out OK (635 ohms IIRC) and puts out a signal when cranked, within spec. in neutral it starts cutting out at 1900-2000 RPM.
Any Ford Techs out there run into this before and have a definitive answer? Thanks.
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