OK, got that item out of the way. does the "stuttering" seem like the ignition is breaking up?? With a max power run out to redline, the ignition system has to be perfect. Plugs can get real hot and break down. Same for rotor, cap and wires. When it kicks down at 45 what are the revs? Other possible things, air starvation, sticking throttle plate, coil. If it runs OK under normal conditions (like a trip to the store), then whatever is happening is a response to the application of full power at high RPM's. A chassis dyno would be nice, but I'd look at at the ignition bits. Just my $0.02 worth..

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE