when you say the modem is 'listed'
do you mean that it is in conflict with another named piece of kit ?
IRQ ?
is it set to configure automatically ?
manual set up is possible ...................

It is possible that the modem is 'old' and it has a default setting in IRQ / DMA etc, in which case you can move the value manually.
The cursor moving ? Hmmm
The alleged conflict could, I suppose, be delaying signal data to the mouse.

I feel the following words should be collected up, rearranged and used in their correct context :

'Straws' and 'Clutching at'