func_timer

1.Firstly, create a good sized room (300x300x300) and place your info_player_start and a light in there. Give your light these values

"targetname"  "splash1"
"spawnflags"  "1"

2.Place a func_timer entity in the map, and give it these values

"target"   "splash1".
"wait"(is how long between firing the target)
"random" (can be set to add variance to the firing, default is 0.)
"delay" (is the time before the first firing when turned on.)
"pausetime" is an initial delay if used with START_ON ("spawnflags" of "1").

3.Lets add a spark sound to the flickering light. Put a target_speaker entity in and also give it the values

"target" "splash1"
"noise"  "world/spark1.wav".

4.Compile your map and watch the light flicker on and off.

Here's a sample map showing the timer example.