S3 Sleep ONly Works w/2GB DDR2

Older Shuttle SG33G5 with Core 2 Duo E8400 running Windows 10 Professional. System works great except for one thing - it won't come out of S3 sleep if I use 4 GB of DDR2. This is a two-slot dual channel motherboard. I have quite a bit of spare DDR2 and I've tried them all, the behavior is the same. If I install two 1 GB sticks, S3 sleep works perfectly, clicking the mouse or tapping a key brings the system out of S3 quickly. When I install two 2 GB sticks, when I click the mouse or tap a key the power light goes on, the fans spin, and I get a blip of activity from the HD - then, nothing. Screen remains blank, no further activity, fans continue to spin and power light remains on. Nothing I've tried on the keyboard brings the system back up or shuts it down.

If I hold the power button until it shuts, off, then turn it back on, it boots normally, except - it bypasses the logon screen, just as I have it set to do when waking from S3.

I've tried every combination of BIOS settings I can think of, S1 instead of S3. tried various DDR2 speeds, looked at all the HD settings, etc., to no avail. Currently, I'm just using hibernation instead of suspend, a little slower, but it works.

Why would the amount if DDR2 memory affect waking up from S3? The system has a brand-new 350W power supply and I looked at all the voltage outputs on an oscilloscope, they're perfect (no ripple or noise) and the voltages are spot on. Any ideas would be appreciated, thanks.