Boot netbsd 2.0 on RiscPC

Hi,

I have a RiscPC, and I want to install NetBSD on it, and now I have a 
problem...

Well, actually I even managed to install it. But my problem is, that it is 
very hard to boot a kernel. Maybe one out of 30 tries actually works. I tried 
lot of different things:

* boot the kernel by just clicking on the kernel icon an let !btnetbsd do the 
rest
* manually execute boot32, in many variants, including from the supervisor 
prompt by pressing * while starting the computer, from a !Zap taskwindow, ...
* switching the cache off/just the instruction cache/...
* removing one SIMM module

But it seems to have no influence. The result is the same. For the install 
kernel, it usually says:

panic: initarm: Failed to align the kernel page directory

But for the RPC_WSCONS kernel it usually just hangs the computer without 
giving saying anything.

But, under rare circumstances, it manages to boot the kernel properly. But I 
am not able to find a correlation between the things I tried out. But if it 
booted, then it runs without problems.

What can be wrong? What could I do so that I can reliable boot NetBSD?

The machine configuration is:

Acorn RiscPC 600
Strongarm rev.K
64MB RAM (2 SIMMs)
2MB VRAM
RISC OS 4.02

Thank you very much for your help!
Peter Teichmann