i got this from blueweasel...it didn't work for me but give it a try...
How to loop Q3 demos:
1)Place the following lines into a standard text editor (notepad):
i got this from blueweasel a few weeks ago .... didn't work for me but give it a try...
timedemo 1
Set D1 "demo demo001; set nextdemo vstr D2"
Set D2 "demo demo002; set nextdemo vstr D1"
vstr D1
(This config will loop the built-in demos 1 & 2 that come with the game.)
2)Save this config into your \Quake3\Baseq3 folder as "demoloop.cfg"
3)From the Q3 console, type "exec demoloop.cfg"
The looping will continue until ESC is presssed.