ADSL switching off/on

Hi,
I've recently put in a new motherboard and now when I'm on the net the ADSL
connection sometimes switches off for a few seconds then back on (same
effect as clicking the 'stop ADS connection' button) ??
There seems to be no pattern as to when it does it, sometimes a few times in
a minute sometimes ok for all the time I'm on ( can't remember if it
switches off when I'm offline).
I'm with plusnet and have a A220(?) internal modem.
The 'enable idle disconnect' and 'disconnect when connection not needed' are
not checked.

Ran adaware and my AVP (Pccillan).
Any idea's what this could be..

RobertJM