One part of the way I managed to often stop oscillating was the random search limit based on these declaration. You may wish to try similar?

  static final int SEARCH_LIMIT1[] =  { 7, 9, 11 };                // 11, 12/13??, 15 works
   static final int SEARCH_LIMIT2[]  = { 9, 11, 13};                // was 13
   static final int SEARCH_LIMIT3[]  = {11, 13, 15};                //
... then before your minimal search..
         i = NumberofPieces(COMPUTER, HUMAN);
         if (i<FEW) SEARCH_LIMIT = SEARCH_LIMIT2[LEVEL] - select(3);
         if (i<FEWER) SEARCH_LIMIT = SEARCH_LIMIT3[LEVEL] - select(3);