Cuttler Hammer makes a good two indivdual circuit AFCI breaker made for 3-wire.
However Vlad it sounds like you have problems circuit or the load. Troubleshooting these can be a real PITA! Contact between another neutral or ground can do that. Most often it would trip right away, but have seen some that remain on until load is applied. Isolate the circuit and check to see if any continuity between the neutral and any other ground or neutral.
Load issues could be anything... Vaccuum cleaner with electronic speed controls, LCD screens, flouresant lighting, dimmers, etc. Many manufacturers have known equipment conflicts that just wont work with the breaker at all.
