Best advice on sleep/wake?

I can easily put system to sleep, but the wake result is variable. After a few minutes, touching keyboard or mouse will wake the system correctly . After some variable time (perhaps an hour or more),I can still hear PC fans running, but the monitor won't come on. To have the screen come on I have to force a shutdown and do a "cold" boot (not restart).

There is a ton of information on the web about this sort of problem, but I post this in the hope that someone on the forum can cut through the fog of conflicting information and either point to a solution or a troubleshooting sequence that will get to the source of the problem.

Just to clarify, you have 2 scenarios:

1. Put system to sleep. Within a few minutes, move mouse to wake system. System powers up and windows resumes normally.

2. Put system to sleep. Wait a long time then move mouse to wake system. System powers up but Windows does not resume.

Is this correct?

In scenario 2, after you do the cold boot do you see a wake event in the Event Log for the initial wake attempt?