Interleaving improves the stability of your line. If your router does not show that its on or off, then your ISP is the way to go (ask them)
I have a router/modem and mine doesn't tell me, but the way that i know when interleaving is on mine, is that i tracert bbc.co.uk
Interleaving off its usually 14ms
Interleaving on its usually 33ms
Basically, with interleaving off and you are suffering from drop outs of connection, that will probably be down to lots of noise on your line. BT's auto detect thingie will slap interleaving on to stabalize your connection.
If that fails aswell, they will more than likely raise your SNR value's and lower your BRAS stable rate.