I think we've all come across this at some point. I know I have!
Usually when the aircraft is hunting in vertical speed, that is a sign that the aircraft is incorrectly trimmed. You may think you have trimmed it out OK, but it won't be.
Check the obvious things. Is the attitude right? Make reference to the horizon using the top of the instrument panel in S&L flight. How high above the top of the panel should the horizon be? Is the attitude correct? That's the first thing.
Secondly, the airspeed. If you've trimmed out for 90Kts but after hitting a thermal you've slowed to 85Kt or accelerated to 95Kt, then the the aircraft is no longer in trim. If you're hunting in airspeed then that's a sign you are incorrectly trimmed.
Third, check the power setting. Do you have normal cruise power set? Is the carb heat on? Have you adjusted the mixture?
I was always taught PAT: Power, Attitude, Trim. When you trim you are trimming for a certain attitude at a particular airspeed and a set power setting. If one changes then it's back to square one and you'll need to re-trim!
Hope this helps. Some PPRuNE-dwelling instructors may care to elaborate? (Apologies if my attempt at explaining is mince; perhaps some FI patter may help!)
Smithy