Signs of a bad logic board?

Hey, all...hoping someone here can tell me how big of a problem I'm looking at...

My Macbook is from Summer 2007 so it's coming up on being four years old. In the last couple months, it's started showing some signs of its age, such as:

1. The USB ports seem to be losing power (self-powered external hard drive works alright when plugged in for Time Machine but USB headset fritzes out about every twenty minutes or so and flash drive won't stay connected more than ten seconds).
2. DVD drive occasionally doesn't read DVDs/CDs but just makes a horrible grinding noise for a while and then spits them back out.
3. Ethernet port seems to be failing more and more frequently; internet connection will drop out for a little while and then come back (sometimes after I restart the comp). Airport seems to be doing this as well now and it may be partially related to just general bad internet service, but it's been happening in two different places with two different providers so it seems to be partially a computer issue.
4. The computer occasionally just...stops. I can't tell if there's a correlation between this and particularly high RAM or processor usage...there doesn't really seem to be. The screen is still there and the mouse still moves but I cannot click or change windows and the computer just sits there huffing away and not allowing me access until I hold down the power button to turn off and restart the laptop.

I'm already planning to get another computer in the next six months or so but I'd like to know...are these signs that the laptop may fail sooner than that? Specifically, is my logic board dying? I know the USB and ethernet ports are both related to the motherboard so this was the first thing that popped to mind but I'd like a second opinion from someone who knows more about the subject.

Thanks very much!

It sounds more like your hard drive is dying not the logic board. There are other symptoms which indicate various problems such as the optical drive, however, there is nothing you mentioned which points to the logic board.
USB port power may not be strong enough to run some of the devices you mentioned, but that can happen even with a new machine.

First thing I would do is make a backup of all your data. Use whatever backup means you have. Don't wait.

Next, run the Apple Hardware Test on your machine. See this LINK. And keep making backups.

It could be the logic board... sounds like a hdd failure... but you could try some things to eliminate the most expensive reasons .
1: Try booting the MB from the install disk go to Utilities>Disk Utility then both Repair Disk and Repair Permissions. There could be corrupt files etc that is making your computer go a bit *wappy* so that could help.

2: Try a fresh install (if you have everything backed up that is), or just wait until you get a new computer and have all your files safely there.

- May as well try things that are not going to cost any money first -

Hope this helps a little

- Simon