Do you encounter unstable connection using your wireless connection on your equipments even when you are within line of sight of the router?
If yes, I will suggest you might want to hard reset your router, upgrade your firmware (if applicable and possible) and reconfigure your wireless setting to check whether does problem persist.
If no, likely cause is due to a poor wireless signal transmission which may arise the intermittent connecting which you are encountering issue. Personally, some recommendation for your consideration.
1. Relocating your Cable Modem to a more centralize place.
2. Install a data point connection to affected area and place another router there on bridging mode.
3. Purchase a wireless extender to enhance the coverage.
4. You may wish to engage their HT services for a on-site survey for their recommendation too.
Lastly, just to confirm what equipment are you holding on currently?
Based on specification from your previous router, the DIR868L should had a better performance. Can you share whether is the location of the current and previous equipment located at the same spot? If no, then it wouldn’t be fair comparing equipment to equipment due to house infrastructure. At the meanwhile, wonder have you tried Eric suggestion? You might also like to ensure your DIR868L is on the latest firmware 1.09.B08 (http://support.dlink.com/ProductInfo.aspx?m=DIR-868L).
Yes, 5Ghz as comparing is actually much weak in wireless coverage. It will be recommended to use 2.4Ghz if the distance is further to have a better and stable connection.