Site runs slow

The first time I load the site it runs fine as I click through the pages. But after the forth or fifth page it starts to run slow and takes ages to load the page. I check with the hosting company and looks closely at the load on the server to check that its not a server issue. I see a few other customers with the same problem and you advised them to install a cache plugin. Seems to have speeded up the site but not solved the problem. Ist this a known issue?