Gigsley

i have been noticing on my ipod whenever i plug it in to itunes that some of the ratings that i put on my songs completely have disappeared. it first started with one song and i thought that i had just forgotten to rate it, but now i'm noticing whole albums are losing their ratings...
it's a fifth generation clickwheel and i've had problems ever since almost a year after buying it. has anybody else seen this and is this something that i should be worrying about?
thanx for your posts