A/T lag...

I've, unfortunately, got A/T. Occasionally, it'll rev a little, then it'll uncomfortably bump into gear a second later, much like the effect you get when you haven't let the clutch out all the way in M/T when taking off. I've checked my ATF level, and it's fine, but I don't really know how to check and see if it's clean or needs to be replaced (I'm used to driving M/T, man do I miss it now). Anyone know what might be causing this?

Could be multiple things. Could be that something is broken inside the trasmission and is clogging the filter. IDK if there is a possibility that the TCU could be causing it? I would take it to a mechanic for diagnosis unless you really know automatic transmissions.....which I don't.

yeah, +1 on the filter, seems like fluid isn't circulating properly

Worst case:

It's probably just old, the clutch discs inside are wearing out and not grabbing like they should (C1 clutch pack to be exact), or the pump isn't pumping hard enough to clamp down the clutch discs like it should (or filter clogged).

AKA

Your tranny is probably on its way out. Baby it and it might last a while. Swapping out the filter and fluid could either speed up or slow down the process of it dying, depending on how regularly the fluid has been changed in the past.

I'll keep babying it, but I've always heard about transmission failure as time progresses... I don't believe that the previous owners took very good care of it; it appears that they raced it (despite the A/T?), yet, I noticed Wal-Mart oil-change stickers (which shows either laziness or inexperience, I'm thinking the latter), so, apparently, they didn't know how to change the oil on their own. It's safe to assume that the transmission fluid hasn't been changed for a while.