HP Printers dropping "offline"

Anyone,

I'm having problems with several HP printers in my office. The problem does not seem to be model specific but rather brand specific. A hand full of my HP printers (LJ4200,4250,4300,4350, 1320, CLJ3800,) will periodically drop off line. You can't  ping them, you can't get to the web interface and the print queue (windows print server) shows the printer is "offline", and clearly print jobs fail. Sometimes, and I know this sounds wierd, if you turn off SNMP on the print queue (ports -> configure port) the printer will show "online" and may or may not print, often it will start printing though. However the problem will return.

The only 'fix' to get the printer printing again is to power the printer off and back on again. Then it will print fine until there is a significant period of time between print jobs - at least 15 minutes.

The problem seems to come and go also. I'll make a setting change (see below) and it will print fine for a day, a week, a month, maybe two, then it's back to the same 'ol same 'ol.

The only common element I can see is that it's always the same physical printer. So, if I moved a 'problem printer' from one department to another it would act up in it's new location. I've worked with HP printers for over 15 years and I'm having a hard time believing that we have 10 or more printers, different models, that are bad with the exact same symptom. For the record we also have several Sharp Copier/Printers (big suckers) in each department and they never seem to have the same problem

Other info and prior troubleshooting:

The effected printers are all over the company's campus and branches. So they are all on different VLANs and switches(some Cisco, some Dell, some HP) and essentially different networks so I'm somewhat ruling out network hardware as the problem.

I've created new print queues on a separate print server.
I've tried setting up the user to print directly to it. (via IP)
I've turned off power save and sleep mode settings (it doesn't seem consistent with the timing anyway, but I did it for fun anyway)
I've swapped jetdirect cards (in the printers that I could)
I've swapped switch ports and switches.
I've forced the NICs and switch ports to every combination of speeds and duplex settings
I've changed IPs (thinking maybe there was a conflict or something..I know it's reaching)
I've put in fancy surge protectors (the kind with the batteries-I know reaching again)
I've swapped patch cables.
I've flashed firmware on the printers.
I've updated drivers to the latest version

None of these things I've done have solved the issue... Anyone else have any ideas?

Thanks,
Brian Holmes