My SMA cbot is overtrading

I created an SMA cbot to execute trades when the SMA crosses the bid price and it works well except for certain times where it over trades within a small range of price any help?

protected override void OnTick()
        {           
            var SMA = sma.Result.LastValue;
            if (SMA > Symbol.Bid)
            {
                Close(TradeType.Buy);
                Open(TradeType.Sell);
            }
            else if (SMA < Symbol.Bid)
            {
                Close(TradeType.Sell);
                Open(TradeType.Buy);
            }
        }