Using other routers on the same line show very little variation on reported SNR values and usually no disconnections - we have tried vigors, d-links and a few others I can't remember, all showing a rock steady SNR value. In the case of a DG834, firmware versions 1.05.00 shows very little change in SNR, but upgrading to 2.10.10 or 3.01.25 on the same line on the same router show wildly varying SNR values.
This is not down to line conditions, its a fault in later firmware versions but we have yet to convince Netgear to investigate further. We have managed to get past 1st line support to 2nd line support in the UK but since then no word from them.
This is also affecting many people on different lines / exchanges and suppliers (BT & LLU). It appears to be more prevalent on lines > 2Mbs speeds which is probably why this was not noticed pre-Max.