I don't know the true mechanics of this, but I've heard answers from some IT people and it seems that it could be two causes.
1, the number of paths or ports for dedicated paths and speed (lacking correct term), may be too small for you now. Most when starting out, pay for only what they need at that time. Then, a few years later after hiring more office people, artist, sales and computers, you clog up speed of traffic. So you need to upgrade.
2, some networks and routers can be set to go to sleep after no activity for a while. Check that.