When the printer is "busy," does it respond to ping? Does it respond to ping when not busy? Are the PC and printer connected to the same switch? Is the printer in the arp cache (
http://www.computerhope.com/arphlp.htm ) when it is busy? When busy, try deleting the printer from the arp cache (assuming it is in there) and see if it reappears in the arp cache after a ping attempt. Does clearing the arp cache change the behaviour?
My next step would be to run wireshark on the computer where you are requesting the print job. This might give some insight into what is going on. Ideally I would try it in at least 3 configurations:
1) Printer disconnected. You should probably see something like an ICMP Host not reachable packet coming from the router.
2) Printer is "busy." Here is the interesting point, do any packets come back from the printer? Does it even establish a TCP connection?
3) Printer is working. Should be a whole bunch of data going to the printer. And maybe some coming back, at least TCP acks, but probably a bit of real data too.
Basically, if packets are coming back from the printer when it is "busy", then it is unlikely to be an ethernet hardware problem. If the printer is completely silent when "busy" then maybe the ethernet is flaky. My first guess is that this is more of a software issue, but hardware isn't ruled out yet.