Game lag every 3-5 seconds

Check the Activity Monitor (in Applications/Utilities/), and watch for something periodically eating up CPU time.

Well, I've got no idea what might be going on, because there's really not enough information to go on. One thing to try, though, would be to create another user account on your machine (if you don't have one already) and try playing the game from that account. If it behaves the same way, your problem is system-wide, in which case you'll have some work in front of you tracking it down. If, on the other hand, it works fine from the other account, you've probably got something wrong with your account. Could be some program that you have running on the other account, a preference corruption problem, or any number of other things.

If you've got preference file problems, which is really only one possibility of many, you can perhaps uncover the problem using Preferential Treatment ( http://www.jonn8.com/html/pt.html).

how much free space on your drives?

When you have activity monitor open, check the drive activity.
Could be hitting the drive and/or VM space a bit too frequently.

And are your drives 7200 or 5400rpm?

On the system partition over 20 GB and on the Data
partition over 50 GB.

It's the standard HDD (Profiler tells: WDC
WD2500JD-40HBC0)

According to the Western Digital web site, the WD2500JD is a Caviar SE 250gb SATA I drive at 7200rpm.

I asked that question b/c some Mac systems shipped with 5400rpm drives, which are slow(er), thus on drive intensive things, like games that load big graphics files, pauses may occur when the player changes "rooms" within a game.

Any other ideas?

With only 20gb free, it's possible your boot drive is simply too low on free space.

In general, X prefers 15-20% of your System drive free (to use as swap space, etc).

On my 200gb drive, once I get below 35mb, my machine gets cranky (slows considerably - lots of spinning rainbows), below 20gb, flakey - unresponsive programs, etc).

Are the game files on the system or the data partition? I would suggest that the problem is that the game is on the data partition and is slowing down the game while attempting to communicate with the system partition. The hard drive is the slowest component of the system.

Personally I have never been a big fan of partitioning but rather than reformat, repartition and reinstall everything why not make sure the game and the system are on the same partition. If that helps with the issue then you cqan decide how to handle the problem.