Its hard to say if the banging around is causing the problem, but I doubt it. I usually wrap the ignition module and secure much like an RX.

Most likely the ignition module is the trouble, possibly the plug boots. 3W had a batch of inferior plug boot insulators that failed prematurly.

Secure the module and run the engine and note if the problem is cured. If not check the plugs and inspect the engine for other possible problems. If you cannot find a contributing problem send the ignition in to Cactus Aviation or Aircraft International for service or replacement.

3W's also occasionally suffer from an air leak at the phenolic insulator between the engine block and carb. The block has been known to warp.

Good Luck.