Help! Tec. problems

[Letum]

Im usualy able to repair all problems on my PC without help, but this one is getting to me.

Every now and again my connection just stops allowing me to view web pages.
I can Ping the web address and still continue downloading something or use a IM
program, but all webpages return a "can not connect" error.

A quick reboot solves the issue.
I use a ADSL modem/router and this problem does not occur in other PC connected via it.

My default browser is Firefox.

any help, greatly appreciated.