I'm also on PlusNet IP Stream Max (but not Interleaved), and don't see the tool to turn interleaving off.
AIUI, the Portal tool only appears when Interleaving has been turned on (BICBW), so that would explain why it is not offered to me, but in your case as you say you are interleaved, surely it should be available to you.
Perhaps the reason you are not seeing the tool is due to the fact that Interleaving has been manually set to "On" by BT, rather than the "Auto" setting actually detecting problems and then setting to "On"? ie perhaps the Interleaving setting on your your line is no longer set to "Auto". Just a thought.
I've seen the link from the post from billford in this thread, but aside from that, I've read elsewhere that the BT system turns Interleaving on if it detects 10 reconnections in a period of less than an hour, so that might be a way of turning Interleaving back on.
Rob
PlusNet Premier Option 1 MaxDSL @ 8128 / 448
DrayTek Vigor2800VG (Firmware v2.7_E38)