I'm not saying GPS cannot be accurate...only that it was not made for this purpose, and may not be accurate for this job.
Radar guns, for instance, often have a given range for which they are accurate, like 0-100, for example, and may not give accurate readings over 200 mph. Depends what model. Similar things may apply to GPS, too. At a nice steady state in your car they may be fine, yanking and banking in a model airplane, maybe not.
Maybe it is getting spurious signals from the ECU or radio, entirely possible, right?
Maybe it is just the momentary acceleration off the bungee, yes?
Maybe someone should make some more runs with some more verifiable measurements, like radar and timers. I'm sure if it is doing 350mph, you will have no problem finding volunteers, and no problem verifying it, and all us doubters will have to eat crow!