Maybe your board is develooping bad solder joints. Recently quite a few of my boards are having issues, some having worked perfectly before being put in storage.

That wouldn't surprise me. The board is a bit warped. There must be thousands of solder joints. I cannot imagine a high discovery rate from visual inspection alone.

Damn. I was going to leave this seller some feedback and eBay returned this, "It looks like you already left feedback or you waited longer than our policy allows."

I must have been one day past the 90 day limit. When it takes 2 months for an item to arrive, 90 days seems a bit limited.

EDIT: wait, it cannot be 90 days. I bought this item on 18 August. Is there a 60 day feedback limit? Here is the original auction link: https://www.ebay.com/itm/163192163925

The recent days I played a bit with this board and its jumpers.
Because I have no SXL CPUs I can not help you with your frequency stability problem explicitly.
I tested the 40 MHz FSB with Am5x86 @160 MHz and experienced no issues so far.
The configuration of this CPU and is not given by the manual,
however only two more jumpers need to be set for enabling L1 WB feature (green) and 4x multiplier (white).

My board is a lemon. Maybe if I let it sit in the box for a few years it will become a lemon pie.

BastlerMike wrote on 2018-10-15, 19:50:

If I remember correctly, the pinout of the 3-terminal solder place was inverted, what means that the regulator is to be soldered on the backside of the board. After some measuring it was decided to solder the IC in the shown way. All this was done years ago, so I'm no more familiar with the details but I think instead of LT1584 any LT108x should also work. My board has an AMI bios installed

You can see it it says Phoenix mate