when I lug the engine in higher gears it stumbles erratically
First of all don't lug it, your causing the engine control systems to compensate for something that should not be happening, it's hard on O2 sensors and catalytic converters, no to mention the stress on transmission internals. Now check the spark plugs see if they show any abnormal signs or worn out tips, if they do replace them, change out the fuel filter. If you still don't care for the way it runs then a "live data" scan would be the next thing to do.
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