The following is from my Tips and Tricks post. See if any of this applies. Let me know what else I can do to help.

Plane Does Not Fly Straight - what could cause this?

Assuming you have not displaced the motor and you are having
problems with the plane turning to one side, check the following:

a) is the wing crooked or too damaged - try a new wing.

b) Check the trim adjustments on the transmitter. They may have been moved from center. Set
them to center and make all adjustments assuming you will fly with the trim
set in the center.

c) Check the tail. The foam is attached to the center plastic brace by small
pieces that punch through the foam. This can loosen up and the tail fin can
move slightly away from the plastic brace in the air which can cause the plane
to turn.

Tape or glue the tail fins to the center plastic brace. Also, look for creases
in the foam. If there is a weak spot, it will
cause the tail to flex causing the plane to turn. mine was creased at the
meeting point where the plastic support meets the tail. Looked fine on the
ground, but it was flexing in the air causing a hard right turn leading to
crashes. Replace the tail.

d) Make sure the moveable surfaces are even with the fixed surfaces on the
tail when the stick is centered and the trim levers are centered. You MUST
check this with the transmitter on and the battery attached. If they are not
even, adjust them with the screws on the control horns. The procedure is in
the manual. RTFM

Note, there is a tiny Phillips head screw on the back of the control horn on
the tail. Tighten it or the spool could unwind while the plane is in the
air, causing a crash. (Guess how I know this!)

e) Check to see that the boom is solidly attached at the body. If this comes
loose, it can move around while the plane is flying causing all kinds of problems. It can also
twist so that the tail is no longer aligned. If the tail boom droops, the plane will tend to nose down.

If you look at where the boom is attached inside there is a pinched area. I
drilled a small hole through the top of that area and through the boom. Then
I put a 4" nylon tie through to help secure the boom. I also put packing tape
around the boom and the back of the body where the boom exits. Between the
two, the boom is well secured.

The Porpoise

When you apply power the plane starts to climb then noses up, then the nose
drops and it does it all over again. The problem is that the tail needs to be
trimmed. There are two screws on the tail. The procedure is in your
instruction book. RTFM