Anyone seeing oscillation on the vertical axis? Tail flutter, tail jumping up and down on a high speed pass?
How were the control linkages, horns and servo gears, any sloop?
Maybe the control horns had loosened in the elevons or perhaps the paper had loosened at the hinge line?
Although flutter at a high speed would usually be heard more-so than cause an oscillation...
I have a speedbuild kit that will get a 4S fan from a HK Stinger MKII so we'll see in a few weeks how it does.
As a report back; my thrust vectoring version took a good many "doinks" during the indoor season. I would do backflips and then try an immediate front flip. It was a hoot to watch but resulted in a few doinks. I went through two MG servos on the mechanism and actually pulled the control wire out of it once. If anyone uses the TV module, you may want to make a thin formica doubler or brass tubing inserts where the pushrods connect to the tilting part. The wires enlarge the holes after only 5-6 flights and results in motor vibration and more slop.
Balancing your props and using low mass props really helps. Remember the servo is fighting the gyroscopic effect of the prop that presses into the hinge (recall gyro effect is 90deg to path of movement) so less mass and less pitch will make it easier on the servo.