RE: Well, That was Exciting! Trim help, por favor.
Many great suggestions to work with... Particularly going to 52.5' lines. That WILL slow your RPM very nicely.
A suggestion to overcome galloping? PIO means pilot induced oscillations. Happens when a pilot gets out of phase with his aircraft. If the plane seems to be going nose-up too fast, it would seem pushing in some down is needed. If the plane doesn't respond just right... - takes more time than the pilot would like to ease over from nose-up back to level, say - he's likely to push in MORE down, about the time the plane has started to turn down.
See how that can get away from a guy?
To beat this, make note of how your forearm, wrist and handle point at the plane when it is flying smooth and level. Lock in that memory. If (when) it starts to gallop, it will probably be PIO, so lock your forearm, wrist and handle like for level flight, and 'aim' a little higher than you were flying level. The model should settle down almost immediately, where your arm, etc., are pointing. "Aim" a bit higher, because the ground is harder than your model, don't give it a chance to eat it.