strange problem with network connection

yes I stuck it in here as I think it is a hardware problem, but not sure how.
For months now I have been having problems with my computer disconnecting from my network, something about default gateway not available.
I am connected via Ethernet through a homeplug.
I thought it was software, but even wireless I had the same problem on the same computer. It is also happening in Linux on the same machine.

Also having problems with USB, it keeps connecting and disconnecting I think it is my keyboard, it uses two connections, one for the LEd and one for the data.

Now that part is out of the way, I will come to why I put it in this section of the forum.
I have reason to believe that it could be a motherboard problem, it is getting on now. could a motherboard fault cause the problems?

I think the PSU is ok.

If it is the motherboard I have to think what to do and which way to go.

IMO it is unlikely that a problem with the motherboard would cause both wired and wireless connections to suffer the same problems. I'd say it was more likely that the fault is in your gateway (router). I would have suspected the homeplugs, but not if you suffer the same problems with wireless.

The other possibility that comes to mind (and could also explain the USB problem) is a faulty or inadequate PSU. That can cause all sorts of strange problems.

Thanks for the reply, I did think about the PSU, but thought it should be man enough it is a Thermaltake Toughpower XT 875W, sure it is getting on a bit I got it in July 2011, I went to a website where i put all my hardware components in and it said I need a 650watt PSU or something like that.

My usb have been fine today, no problems at all, but still have problems with losing connection. it is getting annoying now, certainly with BT having problems as well, I do not know if it is my computer or the internet when it drops.

the home plugs are fine, I put the laptop on them up here and no problem, so it is certainly to do with this desktop.
I know the board is getting on, in fact apart from the CPU, video card and main hard drive the whole machine is getting on.

the thing is if I need to buy new hardware, then I have to think on where I want to go. do I drop Windows and spend a bit of money on a Mac or do I go for a intel based Pc, because AMD seems to be dragging their heals, the problem is I am not a fan of Intel, yes I know Apple uses Intel.