Add to SandyP's post with :
Switch network cables just to eliminate the most simple thing.
Check the web interface of the device right after booting it up if you can get in fast enough and try dropping the network speed of the NIC to the lowest possible setting. Typically on Phaser products this is found under Properties > General > Interfaces.
I have a Phaser 3250DN printer that has started dropping off the network after switched on and printing one or two pages. If I power cycle the printer, it comes back online, but then drops off the network again almost immediately so I can't print any more pages. The printer fails to respond to pings while in this state. The printer is 4 years old but has had >extremely< light usage. We've printed about one thousand pages total and is only turned on when we want to print something so most of the time it's physically powered off.