Are there any settings pertaining to ram speed that you could try? 60ns, 80ns, or the like? Hate to give up on a card but it definitely sounds like you've found the issue. Do you have another system you can test the card in? How about another card you can test in your system? (just to rule out issues with the slot)
Shame you aren't closer, I've got a bunch of 4MB and 16MB GVP simm's from my old G-Force '040 card. Check Amibay though - there was a guy on there making his own GVP-compatible memory just a year or two ago.