Odd Modem Woes

I've got a strange problem with a WinModem under NT4.0 on a P200.

I find that it only connects reliably at high speeds. If I get a 52k connection (which I can fairly regularly) then it's ok. If I get a lower speed one (eps. 46.6k - it hates that) then it goes crazy with CRC and alignment errors and pretty much is useless. A little data trickles through at the start and then more or less nothing.

This is v. bad because it's on an autodial system and I sometimes see that it's sitting open for half an hour with only about a meg transferred.

I've installed the latest drivers and firmwear - no improvement.

Two questions really;

i) Anyone seen anything like this before?
ii) Any idea what's causing it?

I think myself that a better (hardware) modem would probably make all the difference but there's that niggling doubt that something weird with our phone lines. Or perhaps the machine is too slow - but then why's high speed ok?