That's a common Intel Speedstep issue. Swapping power modes from AC to battery resets it momentarily to default. But if it works for you, that's cool. Anything based off of the Centrino platform has speedstep built into it to conserve power when it is in battery mode or generally not in use.
If it's a prefab laptop, such as a Dell or something, you'll have to kill it in BIOS, remove the stepping program, and then scour the advanced section of your power management to get rid of the settings that changed. Sometimes selecting the high performance profile doesn't cut it.
I actually wrote a *.bat file that would swap my power modes (power modes actually have OS signatures), disable Speedstep via software, and set the executable to a "high" resource priority every (in this case World of Warcraft) every time I opened the shortcut. I wrote a guide for my raiding guild at one point, but they decided not to pay their website bill.
I'll have to dig around and see if I can find it.
I have a similar problem with my XFX Radeon 5970 Black Edition. EQ doesn't put enough load on it, so it throttles down it's GPU + Memory clock speeds because it doesn't feel like it's being used enough, which causes some dumb lag (although not enough to make it unplayable). I had to actually force AA + AF to get it to maintain a high enough load to get it to stop. Not too much AA though, as the font gets really blurry in P99 EQ for some reason.
P.S. Another thing to consider if all else fails is the temperature. Most notebooks have thermal throttling on both CPUs and GPUs, so if it gets too hot, it will tone itself down to cool off.
Edit:
Article on Intel Speedstep, Cool & Quiet, and C1E states.
http://www.anandtech.com/show/1975
Vista/7 have additional features that can control CPU speed.
Open the Control Panel, then open System and Maintenance.
Go to Power Options, pick High performance.
Click on Change plan settings underneath it, then click Change advanced power settings.
Go to your Processor power management section and change both minimum and maximum processor states to 100%.
Hit OK
Edit 2:
Here's that batch file thing, but modified for Windows 7. Seems to work just fine for me. Make a new file in your Everquest directory (usually C:\Everquest, since putting it in Program Files is generally a bad idea) and name it something like EQstarter.bat or whatever. Make sure it's something.bat and not something.bat.txt, as that can be an issue if you don't have file extensions enabled.
Put this in it:
start /high /wait /affinity 1 eqgame.exe patchme
That will start the program (eqgame.exe), under a higher priority than normal, while setting the affinity to only use one processor core.
Here is how to configure it to use high performance power settings automatically, then have it switch back to a regular power mode(such as power saver) for normal operations.
1) Press Windows Key + R to get the run box, then type in "cmd".
2) Type in powercfg /L > powercfg.txt
3) Type in notepad powercfg.txt
The one you are currently using has an * next to it. You want to find the "High Performance" or "Max Performance" out of the list, and the one that is currently set to. Example:
Existing Power Schemes (* Active)
-----------------------------------
Power Scheme GUID: 381b4222-f694-41f0-9685-ff5bb260df2e (Balanced)
Power Scheme GUID: 8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c (High performance)
Power Scheme GUID: a1841308-3541-4fab-bc81-f71556f20b4a (Power saver) *
Once found, adjust the batch file we made earlier with your settings. They will be different than the ones listed above, so be sure to use yours and not the ones listed here. For example, mine is set up kinda like this:
powercfg -S 8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c
start /high /wait /affinity 1 eqgame.exe patchme
powercfg -S a1841308-3541-4fab-bc81-f71556f20b4a
8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c is "High Performance", while a1841308-3541-4fab-bc81-f71556f20b4a is "Power Saver". So while gaming, it's on "High Performance" mode: 100% CPU power, no underclocking (provided you adjusted the settings in the high performance profile previously). When not gaming, it's on "Power Saver" mode -- 5% to 50% CPU power, saving precious energy and letting laptops increase the battery duration.