So, I am having this problem with a jet I am working on.

It is a 'What if' jet. Apparently Australia has not actually built it's own fighter jet from the ground up.
There is the 'Avon' Saber of course but that was a heavily modified Saber due to use of a different engine.

But I digress.

After air launch (no wheels on it yet), power up, level out, everything is good.
Roll check, left, right, fine, nice fast, tight rolls.

Loop.... umm yeah. Here is where the problem is.
A lot of up elevator causes seemingly random 1/4, 1/2 rolls to the left AND right.

I have hit this problem before and it bugs the heck out of me every time, because I can never remember how to fix it.

The jet has a conventional wing set up, slightly swept back main wing, no dihedral, rear wing the same, single tail, fully upright, engine mounted in the rear (BF300).
No XML done to it as yet, except for the nose being heavier than normal (x4) to counterbalance the engines weight.

Any help is gratefully appreciated.