I would have it apply to Megistaride, but maybe raise the level of boost all stats to balance it if necessary (or drop the item's store price, or both). Plus, it would remove buffs individually at random. Maybe if one of four Megistaride buffs was removed, your HP could drop by 75% the value it normally drops each tick, for example.
Also, I wrote the chances as "low" and "high" rather than percentages to prevent assuming what reasonable chances would be.