Problems going into sleep mode with Windows 7

[C E Jones]

Sometimes when I try to go into sleep mode in Windows 7 RC something goes wrong. The power light doesn't go off as it normally does, and when I try to power back on it won't boot up--instead the power light goes off, then comes back on and goes off again and keeps doing that. The only way to get it going again is to switch of the PSU, switch at back on, then power up. Then it wakes up from sleep rather than booting up! Has anyone seen this problem before? Any idea what's causing it?