There are a few things could cause this. Your DSL modem may be malfunctioning. If this is the case you should call your ISP and tell them to replace your modem, or, if the modem is yours, either have it serviced or replace it.
In order to determine whether the problem is inside your home or outside, take your modem to the NID (Network Inteface Device) and connect directly. This eliminates any inside wiring.
If you are able to sync up fine outside of your house at your Telephone box (more commonly known as the NID, Network Interface Device) Then your inside wiring will need to be inspected. You will have to notify your telephone company and tell them to fix the problem. Note this will cost you money because it's outside of your ISP/DSL service.
Your telephone line could be losing quality. Line noise, electronics on the line, or a short on the line can break up a sync signal. You will have to tell your ISP that you keep losing sync and want to have your line checked for problems. They should open a trouble ticket with your ILEC/CLEC and they will inspect the lines. In most cases they can find the problem this way.
Also read About DSL for lots more information