It is not clear from your question if you are getting disconnections from cables or from wireless connections.
If the problem is a stuttering wireless connection, there are many possible causes, from a damaged router to a bad position of the router in the room (if there are too many walls the connection can be very difficult).
If you have instead frequent disconnections from the telephone line, then possible causes are issues on the line or a faulty modem.
In these days I was looking for a modem-router too, and when I read for users feedback in various stores I found that some models are not usable in some countries.
For example, take a look at this (scroll down the page to read for user feedback).
If I'm not wrong, however, the Linksys WAG320n should be able to manage correctly both protocols, so the first thing you should do is call tech support of your internet provider and ask them if there are problems on the line.
If the line is working, then check if the modem and router are correctly configured.
Too bad, for what I know, there is no software to check a router to see if the hardware is damaged or not (like tools for hard disks for example), so the only way you can check if the router is the cause of problem is trying with a different router. Maybe you can ask a friend to borrow a router to run a quick test