is it exactly once every 80mins? seems strange it would be so precise, that would indicate that either the service provider is dropping the connection after a certain time or there is something causing interference nearby that is making line noise and the connection drops
If the router is just randomly dropping the connection things to try are, connecting router direct to master socket not an extension, if you have to use an extension make sure it's a good quality one and installed properly, it's also often worth changing the rubbish ADSL line filter that came with the router to a decent one, ADSLnation make about the best filters.
If you can get to the admin pages if the router have a look round for the line details, the figure to look for is the SNR margin, generally the higher the better for this figure but anything over 6db should be ok, below 6db and there is a good chance the line will be unstable.
finally 2wire routers are not generally considered to be that good at hanging onto a connection on noisy lines, netgears are considered to be among the best for this.