D-Link Switch

Question

We have a d-link DES-1226G 24 port switch. When we connect it up everything functions OK for about 20 minutes. We have bout 20 computers connected but only 5 are active. After a short time every light on the switch turns on (rather than just active ones blinking) and nothing works. We power the switch down and then on power up the same thing happens. Works for about 20 minutes before lighting up and freezing.