My prims keep disappearing from view!

I'm building a shop on some land I've just bought. Problem is the front of the building (a 2 prim wall) disappear depending where I'm looking from, showing the inside floors.
I don't think I'm doing anything stupid.

Any help most appreciated.

Here's some nerdy stuff from the viewer help menu:

Second Life 1.21.6 (99587) Oct 14 2008 17:42:25 (Second Life Release)
Release Notes

You are at 254347.5, 260363.1, 35.8 in Bhima located at sim4350.agni.lindenlab.com (63.210.157.254:12035)
Second Life Server 1.24.10.106829
Release Notes

CPU: AMD (Unknown model) (1808 MHz)
Memory: 2048 MB
OS Version: Microsoft Windows XP Service Pack 3 (Build 2600)
Graphics Card Vendor: NVIDIA Corporation
Graphics Card: GeForce 7600 GS/AGP/SSE2/3DNOW!
OpenGL Version: 2.1.2

libcurl Version: libcurl/7.16.4 OpenSSL/0.9.7c zlib/1.2.3
J2C Decoder Version: KDU
LLMozLib Version: [LLMediaImplLLMozLib] - 2.01.20735 (Mozilla GRE version 1.8.1.13_0000000000)
Packets Lost: 472/41010 (1.2%)

Hi Tracey, I am there right now. The textures on your floors and walls have alpha channels so it's the sorting glitch I think. I don't see any holes that need alpha channels, so can you upload new textures without alphas? Or get new textures if you bought these ones?

Thank you so much for your help.
Didn't know about this glitch, will have to rethink how I build now.

There was a discussion about this that might help a bit:
/8/9f/250234/1.html