Right now, Crosman offers 2 basic rifles that I know of - the 795, which is more of a youth gun, and the Quest/Phantom series - which are essentially the same gun with different stocks.
Assuming that your rifle was/is a Quest-based gun, I'm not surprised at your results. In my experience there's 3 things that can booger up scoping one of these... assuming you're starting with a proper airgun scope.
The first - and common - is inadequate mounts. If you're using mounts made for a .22 or mounts that came on something like a Daisy Powerline 3-9x32 scope or its Crosman equivalent, they will tend to creep unless there's a good scope stop used. This will be a significant problem on any strong spring gun, BTW.
With break-barrels, there are also issues with the actual pivoting breach mechanism.
The first is when the lockup is inconsistent... for example, if the pivot has significant sideways play it will never shoot well with a scope. On one shot the barrel will be straight, on the next it'll be pointing left, and so on. It may still shoot well with open sights because the sights move around with the barrel, and therefore are always lined up with it. But the scope is not mounted on the barrel.
The second is when the lockup actually moves while shooting. When you shoot a breakbarrel there is a force exerted to push the breach open... if the lockup spring is a little loose, the barrel will move vertically an unpredictable amount. This results in the shots walking up and down in a straight line.
Both my Quest and my B19 suffered from both problems. Both issues are usually fixable by tightening the pivot bolt and/or shimming the lockup detent spring.
Gamo breakbarrels, in contrast, tend to have much better lockup than some other inexpensive guns. Their pivot bolts come tight and stay tight - in fact, they are a real bear to remove! And their detents are nice and strong. If you're looking to scope a breakbarrel, I don't think you'll have any trouble with a Gamo.