Generally FPS loss is a result of simply too much geometry that must be drawn to the engine.
Does it only happen when you look at a certain area?
For instance, if you made a map with many, many, buildings, then flew to the top of the map and looked at them all at once, your FPS would take a hit and drop.
When your map is running, type developer 1 (enables developer mode, if it isn't already), then type r_speeds 1.
This will cause many numbers to start appearing at the top of your screen very quickly.
It should look a little something like this.
<img src="http://www.angelfire.com/dragon/dragondude8/r_speeds.JPG">
The numbers are labeled with FPS (Frames Per Second), ms (your ping, always in Milliseconds), wpoly (World Polygons, polygons currently being drawn to the world by brushes created in Valve Hammer Editor, basically the geometry of the map), and epoly (Entity Polygons, currently being drawn as a result of entities).
As you can see, my FPS at this time is steady at about 60, and it is steady because the wpoly count is well within normal bounds.
Epoly usually isn't as much a factor as wpoly, but having very high epoly counts can also hinder FPS.
I always go by the rule of trying to keep wpoly under 600 or so at all times, though sometimes it can't be helped.
Try this. Is your wpoly count very high?