Problem with FlightLine F4U Corsair
 

Hello,
Idk if this is in the correct place, so if it is not the correct sub forum to post I apologize. Recently I have gotten the new Flight-line Corsair, and I've had an issue that I can't figure out how to fix. When flying the plane for the first time for trim, the elevators seem to refuse to trim out. During flights trying to get it trimmed fully to maintain a neutral state, the plane will fly straight for about 5 to 6 seconds before suddenly moving violently in the direction of the trim (if the trim was to bring the nose up it will violently climb up, and visa versa when down). This is the only area that does this, and on the ground everything is working. The CG has been tested several times and its balanced per the instructed CG location. I have ran through everything I can think of to get the elevators to trim properly, so I'm out of ideas on fixing this issue, and it makes it unflyable due to violence in pitch change while trying to trim the plane. If anyone has any ideas on how to fix this, it will be greatly appreciated.
To give context to the flying conditions of the plane and environment of the flight: No wind at ground level with about 3 to 4 mph consistent winds at an altitude of about 300 to 400 feet with no shift in direction during flight, Plane was flying on low rates (55% on both elevator and ailerons with +15% expo) at 55-60% throttle for just cruising, control surfaces show no signs of over-input or glitching.

Check your linkage, sounds like either a linkage problem or a bad elevator servo

Would there be a way to test the servo? on the ground it shows no signs of glitching or issues under normal checks.

Put your finger against the elevator lightly and move the stick, also when centered see if there's slop in the elevator by moving the surface with your hand.

Also, make sure the servo is plugged in all the way into the rx and the extension is seated tightly if there is one.

alright thank you, is there any other issues that could cause this that I should also check for?