[tor-relays] Relay reboots every 15 minutes

Starting about a week or so the number of connections raised rapidly to 18000+ and since then my middle relay reboots every 15 minutes. Lowering the relaybandwidth to a few MBytes partly solved these reboots. Before these unplanned reboots the relay has run for months at 20 - 40 MBytes traffic without issues. The number of connections now is around 11000 per relay. How can I prevent these reboots?