Where are your ailerons driven from? If they are strip types driven from the root then that's a long airleron to keep stiff enough to prevent flutter.

Ailerons made of balsa? Here agian lots of flex. Actually I'd say, without seeing it... , that this is the most likely cause. Thin airfoils flying fast are always going to be flutter prone.

Possible cures. Lead like Will is suggesting may help but it's still behind the hinge line. Another fix can often be to stiffen the surface up enough that the new flutter point is beyond the speed teh model can fly at. Off the top of my head I'd say the easiest fix would be new ailerons made from spruce. Or strip the balsa ones and cover them with light glass cloth WITH THE WEAVE ON A 45 DEGREE BIAS. Note the capitols. You are adding a stressed geodetic skin here so the weave must be at 45 for it to work. Cut strips on the bias like this and just overlap by 1/4 where they run short if needed. Use polyester finishing resin or epoxy for adhesive. Or if you're feeling rushed and rich use wax paper and thin CA. Press the surface up against a flat surface over wax paper and wick a little under the bottom surface. Work your way out to the tip and then flip and start over. The cloth should wrap around the leading edge from lower to upper in one piece. We won't bother making you wrap it at the trailing edge.

The spruce surfaces are sounding better aren't they....

There's one last possible fix that doesn't require you cut into the model. Horn balances. Little lead blobs that are soldered onto a wire that is then attached to the surface on top or bottom and extends forward. I had the same problem on a little 1/2 A racer type. Buzzed like crazy in dives. I went the easy way and stuck on the balance horns It was ugly but it did the trick.

The idea here is to get the CG of the surface close to or ahead of the hinge line. That is what Will's weights are trying to do. But the horn is more effective. Put them out at about the 60% mark along the ailerons. To avoid harmonics you don't want them at exactly the halfway point, 1/4 point or the tips. If you want to be really techy put them at the 58 or 63% point. That should solve any harmonic problems. And don't skimp on the weight. My little 1/2A used a 1/4 oz. Your model should probably use at least 1/2.