As an side to the good information shared by others here, I'll point out that an exception tends to exist in any situation where you learn spells by virtue of your race; drow and high elf magic traits, for example, grant spells that are controlled by Intelligence, regardless of whether you have another casting class or not - the idea being that you learned these in the wizarding style of academia as part of your upbringing. Another example is Genasi who, if their elemental type grants them a spell or spells, use their constitution as their casting ability for them.
Spells that you get from magic items usually either tell you to use your own casting ability and DC (such items usually require attunement by a spellcaster, so they can do this, knowing that you have a casting class to work with), or they will supply their own attack bonus and save DC as part of the item (for such items that don't require attunement or can be attuned by anyone).
We don't have magic item attunement in the game currently, so formally speaking, all magic items that cast spells should be using a fixed item-set attack bonus and DC.
Currently in game, there are quite a few bugs; there are several spells that use the wrong casting ability, regardless of your class, and several items that are odd in their behaviour related to their spell casting. The advice above, form the other posters is sound and that's what you should follow - where it diverts from that in game, treat it as a bug and report it if you can spare the time.
