Unfortunately Cure indeed looses it's potency when it's used by another class - MND has a lot to do with it, but also CNJ does get traits that also boost healing skill specifically for that class, meaning as you increase in level Cure will actually appear to get worse healing wise. You could try boosting your MND stats with items or equipment, but it wouldn't really help too much I'd imagine.

Try also equipping the ability Convalescence from GLA as that boosts the power of healing spells cast on you by a fair amount while the buff is active (again, GLA gets a trait that increases the potency this ability gives, so cross classed it's not as effective), but every little bit helps.