I've had a Tualatin-compatible Shuttle AV18E in storage I've been intending to put into a multi-purpose DOS/win98SE retro gaming box that I finally got around to testing recently.
It's a VIA Apollo Pro 133A based board, and I know VIA boards don't hold up to the performance of contemporary Intel chipsets, but I already had this on-hand, it's a reasonably flexible and Tualtain-ready, and it had a pretty good track record as our faimily's shared multimedia PC some 10 years ago (and then my younger brother's PC for a while after that).

It seems to work fine (1.4 GHz PIII-S and all), but fails to recognize any SDRAM module over 128 MB, though it works fine with single and double sided 128 MB sticks. At first I thought the DIMMs I was testing were bad (which I'd gotten as-is from an a scrapper), but it fails with the handful of old 256 and 512 MB sticks my dad still had at home (including a registered one, but that's more to be expected). -To be clear, the 256 and 512 MB modules don't add to RAM when coupled with smaller densities, and the system won't even POST if only 256/512 meg sticks are installed. (error beeps sound for no RAM installed)

It works perfectly fine with all the 32, 64, and 128 MB modules I've tried, and does so for all 3 sockets.

The manual mentions support for 256 and 512 MB DIMMs in all 3 SDRAM sockets (and up to 1.5 GB total RAM), so I'm not sure what's going on here.

Has anyone here had experience with AV18 or AV18E boards, or similar problems on similar boards?
Could it be a BIOS issue or some weird jumper I've overlooked?

As it is right now, I've got a pair of 128 MB sticks in it, one PC133 and one PC100, but both using 7.5 ns SDRAM chips (so 133 MHz rated) and it seems to run fine even at 150 MHz (at default SDRAM timings), and 256 MB is good enough for pretty much anything I'd want to run, but I'd rather not be stuck with that.