Since the problem started when you changed the tail, I would be suspiceous of the tail not being properly alligned.

Check the manual for stalling and porpoising. The angle of the tail, not the control surfaces but the tail itself, is critical to the plane flying properly.

I would guess the front is down too far, or the back is up too high. The orange screws will have to be turned.

In theory if you loosen the front screw and tighten the back one, it raises the front of the tail and lowers the back. You do that, then try a flight.

Before you do that though, you must get your tail surfaces back to proper allignment - even with the fixed part of the tail.

If the front of the tail won't stay "up" you may need to slip in a think piece of cardboard or something. That is what I had to do.

RTFM

Here are my tips on dealing with a plane that won't fly straight, but I think when you put the new tail on you did not properly trim it.

I have an Aerobird with over 100 flights. I have taught several people to fly
their Aerobirds. Here is the procedure I tell people to follow to get the
plane to fly straight.

If you are having problems because of a bad crash, so let me just offer this
as a starting point.

Unless you have removed the white foam that sits between the battery and the
electronics, ignore Center of Gravity issues for the moment. Do all of your
flight testing with the 6 cell battery. The 7 cell makes it nose heavy and
will change the way it flies. When I am trying to fix a "bird" plane, I use
the 6 cell only.

1) With your transmitter on and all trims centered, and your battery connected
in the plane, but with the motor off, look at the control surfaces on the tail
from the back. Are the movable parts exactly even with the fixed parts? If
not then you are going into a turn the moment you launch assuming the motor is
straight and the tail is straight. More on that later.

Normally, these surfaces have to be perfectly aligned.

2) The boom between the pod an the tail - is it solidly anchored or can you
move it around inside the plane. If it moves, it has broken lose. This must be
fixed. It will either sag causing the nose to go down or it is twisted causing
the plane to turn right or left.

3) Check the tail, especially by the rigid plastic near the boom. Are there
any creases? I had a problem with my Aerobird that caused it to turn to the
right so badly that it crashed because it would go into a spiral. I tried
everything. Turned out there was a crease in the tail that caused the tail to
flex under pressure. On launch, this could take you into the ground.

4) It is possible for the tail to shift from a severe nose crash. There are
trim instructions in the owner's manual. AFTER you have checked the other
items and
fixed or found them to be OK, try trimming the tail for more up or down force.


Other points to be aware of:

When the motor is running, more air moves across the tail so that you get a
faster response for turns. The slower the motor is running, the slower the
plane will respond. When gliding, response can be very soft.

Make sure you are launching into the wind - directly into the wind, or the
plane will be turned by the wind when you launch. Same for landing.

If this doesn't help, I invite you to post photos. These are what I would
want to see:

view from nose to tail - level with the top of the plane - to check alignments
view from tail to nose - same reason

Remove the wing, lay the plane on a table with the tail hanging off the end.
Place a ruler under the body extending to the tail. I want to see if the boom
is straight

surface alignment - make sure your trims are centered, the radio is on and the
battery is plugged in.

view from the tail at tail height - full left command
view from the tail at tail height - full right

If this is an Aerobird:

same - full up
same - full down

Good luck and I hope this helps.