I can't imagine your internet traffic being so fast that it actually slows down your internal network to a halt.

The TTL is the number of hops a packet can pass before it being dropped to avoid rogue packets passing through a network till the end of time. I can't imagine it posing a problem.

You do have different types of cables, but you mentioned before you use Cat 5. which is good enough. I can't imagine you picked up any older cables in a store recently So I reckon the cables are good if you've tried replacing them.

I'd go about checking things like:
- try another router see if the problem persists
- try a switch in stead of a router
- try different NIC's
- do the pc's have more than 1 nic? If so, disable 1 of them

Just some things you may try....

GL.