Powering on PC causing modem dropouts

G'day everyone!

Hopefully someone can give me some assistance as I am all out of ideas.

I have an oldish desktop (about 5 years) that all of a sudden has developed a massive problem,

I am using separate SSD's with WIN10 on one and Linux Mint 18.2 on the other and a 2TB HDD for data, I simply boot into the OS I want when it loads.  Been doing this for quite a while now no problems.  We have about 5 computers in the house (between 2 of us - so not all being used at once).

However, for the last few weeks or so, when I turn on the computer it POST's correctly and it doesn't matter what OS I choose it causes the modem/router to drop out completely.  It re-syncs for 30 secs / 1 min / 5min / etc then drops out again. Wired network.   TP-Link TD-W8968.  This only occurs when I turn this one machine on.

Motherboard is Gigabyte GA-Z77X-UD5H - with 2 onboard network adaptors.

I have done the following:

* Reset/rebooted modem countless times.

* No IP / MAC conflicts that I can see.

* Virus / Malware checked all drives with both Kaspersky and Malwarebytes after removing the drives and using dock.

* Current network drivers.

* Unplugged the Ethernet cable(s).

* Unplugged peripherals except monitor.

* Unplugged all peripheral except the box.

* Unplugged the drives - no peripherals except box.

* Removed CMOS battery and left off (removed power cable) for an hour. And the real kicker...

* Took the now driveless box to another room and powered it up ... POST's and then modem dropout... WTF?

It's been a long day and I'm sure I've missed something obvious to try... any help will be greatly appreciated.

Dan

router is failing if the tiniest load causes it to go down.

Not a pc issue

Figured it out (eventually)... after all the fiddling I was confident it wasn't a network issue as such but a power issue.

I plugged the modem/router into the UPS thinking it might be a house power issue, but same result...

Then it clicked... or in this case ticked... my PSU has been steadily ticking away for a few weeks now and I had just gotten used to the sound as it didn't seem to be causing any issues.

Took the PSU out and plugged it into a PSU tester and ... FAIL ...

Luckily I had an almost new Thermaltake PSU on hand and plugged it in... no modem/router dropout... put all the bits and pieces back in the PC, powered up and voila! No drop outs....

Taking the 1-2 year old Antec Neo Eco 520C back to the shop for a warranty replacement.  I was going to just stick with the Thermaltake that's now in the PC except that it only has 3 SATA power connectors. Running without a optical drive at present (not that the're that useful nowdays).

Problem SOLVED.