Rear tire feels unstable in turns

[gstam4]

hey all - seems my leaking-gas and other mechanical problems have been fixed, so i'm moving on to the next one on my list.

i have tires with plenty of tread, and the air pressure is good. they're a little on the old side (mid to late 2008), but don't show any signs of rot. if i decide to keep the bike past the summer, then i'll probably replace them.

when i take a turn with even moderate speed and lean the bike a little (i mean really a little - even if i'm doing a right-hand turn onto another street at 10 mph), the back end/tire feels 'unstable' - as though it's sliding, even though i know it's not. it's hurt my confidence in turns, because i'm worried the bike will slide out from under me. i don't have any stability problems riding in a straight line, other than getting blown all over the place by the wind.

assuming my tires have proper pressure and aren't themselves the problem, anyone know what it might be? i saw 'trail number' mentioned online somewhere, but not in relation to a gz.