Wired router dropping connection

Hi,
I recently set up a wired network within my house. All wiring is okay as it was installed by an electrician.
I have run a normal DSL modem serving one computer with no issues but obviously I needed a router to serve my other PC.
When I connect the router, the connection continually drops for a few seconds every few minutes on both PCs. If I remove the router and run either PC from the modem no issues.
I understand this is a common issue with wireless routers as well.
I am running Windows XP.
Can anyone offer any advice as to why this is happening and what the solution is?
Many thanks,
Chris