ZipGS only stable with 32K - why?

[spec@vax2.concordia.ca (Mitchell Spector)]

   About a month ago, I received a pre-upgraded 15 MHz/64k cache ZipGS
board. The original owner claimed it was perfectly stable with the
modifications he made and the type of static RAM chips picked. It had
run on his ROM 3 for well over a year with a stock powersupply and a
few cards. However, when I installed it in my system (also a ROM 3 with 
stock powersupply) I began to experience lock-ups and freezes.

   I decided a weak/overloaded powersupply might be the cause, so I
purchased one of Stephen Buggie's 150W PC supplies. Unfortunately it
made little or no difference, the ZipGS would continue to lock-up!
Usually it runs perfectly stable for several minutes, then suddenly
a lock-up occurs, then yet another and so on, until finally it gets so 
bad clicking on a Finder icon will cause an instant lock-up (mouse pointer 
moves but background frozen -- yellow/green LED on card at this point gets 
very bright and stays on). I tried swapping in another oscillator, slowing
the board down to 12.5 MHz, though the same problem still occurred. 

   The only solution I found, is flipping the card's DIP switches so
it's configured for 32K cache (effectively, this tells the card to
ignore the upper 32K of cache). My question is: _why_ on earth does
the board run with 32K cache but not 64K cache? I doubt the chips are
faulty, I've swapped the TAG chips around, still no difference. They're
also the correct speed (TAG = 15ns, DATA = 45ns) and _did_ function in
a similar IIgs setup without causing lock-ups. At present I'm running
the board as a 15/32, but I'd like suggestions on how to get it working
as a 15/64. Note 15 MHz works absolutely fine, it's the cache that is the
sole problem (I'm running stable now with it as a 15/32).

My system consists of (listing everything, just incase :)

ROM 3 CPU, Buggie 150W PC supply, System Saver IIgs (cooling insides), 
Apple 3.5, Apple 5.25, Apple SuperDrive, TMS Shadow HD w/730MB Quantum 
Lightning, USR Sportster 14.4k, Mach III joystick, Quickie scanner, 
standard IIgs keyboard and mouse. Also unshielded Bose speakers,
AppleColor RGB and ImageWriter II, but I doubt the latter could
be linked to any problem.

[Slots]
1: 
2: Audio Animator (currently MDIdeas cards plugged in)
3: ZipGS 15/64
4: ComputerEyes GS
5: Quickie interface
6: Apple II 3.5 Controller Card
7: RamFAST rev D, 1MB

Mem Exp: Sequential Systems RAM-GS 4MB

Mitchell Spector
spec@vax2.concordia.ca