Drive keeps dropping out of RAID-1

Hi all,

I have a recurring problem with a disc within one of my RAID drives. A partition keeps dropping out. I can add it back it, the RAID rebuilds but a week or so later drops out again.

I have 2x40GB SATA partitioned (sizes approx) as follows:
/dev/sda1 100M
/dev/sda2 35GB
/dev/sda3 4GB


/dev/sdb1 100M
/dev/sdb2 35GB
/dev/sdb3 4GB

I have then defined 3 RAID-1 devices using mdadm

/dev/md0 /dev/sda1 /dev/sdb1 /boot
/dev/md1 /dev/sda2 /dev/sdb2 /
/dev/md2 /dev/sda3 /dev/sdb3 /swap

/root is on a 4th RAID-1 from 2x500GB SATA drives.

It is /dev/sdb2 that keeps dropping out but NOT the other partitions on the same drive from /dev/md0 or /dev/md2

Any thoughts - Do I need a new drive?

Many thanks.