One thing to take note of is how often the disconnects occur, and if they are noticeable to you. Almost every DSL product will disconnect at some point, however, it can be that your ADSL service is dropping quite frequently, but is happening either at times when it is not noticeable by you, or that the drop is so short, that it would appear that an internet page may 'freeze' for a few seconds, making it look like the connection was slow for a short period, but in fact the PPP session had disconnected momentarily.
What you should be looking to do for your investigation, and to help your ISP to understand the issue, is check your router logs for any PPP CHAP authentication loss/failure/success and note down how many have occurred and over what period.
This will give you an understanding of how often your ADSL service is losing connection, and for how long, so you can determine if synchronization is lost, or whether it is simply the PPP (internet) connection being lost. If you connection is dropping for a matter of seconds (10 or less) then this would mean that your router is only losing PPP connectivity. However, if the loss is occurring for longer than this time, then it is more likely that your ADSL synchronization is being lost.
Intermittent connections are quickly identified by your ISP if you complete a couple of quick steps such as connecting into the nte5 test socket (located behind the front cover of your master BT socket) and replace the micro-filter. If these are completed and your router still shows disconnects, then it is likely to be a fault on the BT network, which will need identification/resolution by a BT Openreach engineer.