A big clue to the nature of your problem is whether you're making trim adjustments back and forth, or whether it's always in the same direction.
For example, if you're always tightening the cable it's probably slipping, or the housing is extruding through a ferrule as the prior posters suggest.
If you're adjusting back and forth, it's neither of the above, and probably sticking slightly within the housing or the lever mechanism isn't as crisp as it should be.
Next time it's out of trim, don't try to fine tune it. Instead pluck the bare wire away from the downtube like a guitar string. If that works, it's definitely a sticky problem in the cable or lever.
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