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Given the initial description.... Guiding frame remains unchanged for 2 hours but image moves.
If it rotates it's polar, if it drifts in one direction it's flexure.
My guess is it's flexure, this can be due as previously stated a number of factors.
Some common ones are......
Focuser is not rigid enough for the weight of the camera, a dslr is much heavier than an eyepiece, the barrel of the focuser in cheap scopes will have some movement, hatbis why people buy precision focusers such as the feather touch
The tube of the scope itself is not rigid enough to support the weight and as it rotates through then equatorial axis changes in the angle of the camera causes movement
Less likely is the rings are not solid enough
The plate/ bar whatever you use is not rigid enough, I use a solid alloy plate not a thin bar.
Wherever you have some form of adjustment, there is always opportunity for some movement, be it a focuser, scope anchor points, rings, you would be surprised at how some things just seem to come loose, check your entire imaging train EVERY time you image, don't assume because it was tight last time you used it , that it remains so.
I speak from experience in these matters and some scopes.... Particulary those designed as Dobs were not specifically made for imaging and whilst they're fine for their intended purpose they need quite some tinkering to get to perform as an imaging scope.
Post an image of your setup and maybee it will be more obvious.
Multiweb has given you a good tip, the off axis guider will solve the problem as the scope will move but so does the guider.
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