Please read here about how to use decals properly:

http://manual.3dgamestudio.net/ent_decal.htm

When particles disappear when you see a decal shadow, your particle number is too small. Either limit your particles and decals or increase the particle number.

Z-beating and elements appearing through walls are caused by a wrong clip_near/far range. Use careful level design and don't tamper with the view range settings. Otherwise increase clip_near and reduce clip_far until the problem is fixed.

I think this should be easily fixed by yourself - if not, please post again and we'll help.