I have an XT that had the same problem.
First thought: be sure the shelf that locks the barrel to the receiver down at the bottom of the action is not lubed very much at all. It needs next to none. I know this sounds weird, but other Citoris I've had were sensitive to too much lube on the shelf, especially if there was any sort of water in the air that could condense or fall on the shelf. When the lube and water were present, the lever popped to the right when shooting. I use next to nothing on the shelf insofar as lube goes. Just a bit of a sort of gun "dry lube" like the kind used for Remington semi-automatic actions.
Second thought: the top lever spring needs replacing. This, more than the issue with too much lube on the shelf, was my problem. When the top lever spring was replaced on my gun, it stopped popping open.
Third thought: the shelf itself might not be completely square if you've had any work done on the action. My trapshooting buddy had an XT with this problem. Replacing the shelf solved the problem. It wasn't noticably out of square but somehow, it was enough.
My guess is maybe problem one, very likely problem two and unlikely problem three. Bet if you replaced the top lever spring, your gun would be fixed. Good luck.