XSi/450D stops shooting

Hello folks.
Usually when I'm taking pictures, I'm taking my time; tripod, cable release, etc. The camera works fine and never balks. On only 2 occasions (today being the second) I've gone out to take pictures of my wife, and both times run into situations where I start shooting fast and the camera balks. This is what it looks like:

I'll take a couple shots, move a little, take a couple shots, maybe reframe, take a few shots, change location, take a few shots... no problem. Then after awhile, I'll see something in the viewfinder that's just right, and I'll start shooting and not stop. Well, the camera can only do 6 sequential shots before it has to empty the buffer, so it stops. Okay, so I have to wait, then it's ready to go again, and I go back into "take a couple shots, move a little, take a couple shots, etc" mode. Camera's working okay. Then sooner or later I'll again see something that's just right, and again I start shooting and hit the 6-sequential-frame limit and have to stop and wait for the buffer to empty. This is where the problem hits. After this, I can't do more than 1 or 2 shots before the camera refuses to fire and I have to stop and wait for the buffer. I sure don't get my 6 sequential frames anymore. Just when things are getting good, the camera's acting like it's through for the day.

The first time this happened, I had taken 50 to 100 shots when it started balking on me. Today I had taken at least 100 when it started. When this happens, the camera is noticeably warm, but not hot. Battery seems to be fine, still shows half after we got home. I was using a 4gb SD card; it said it had room for about 50 shots left after shooting 156. The camera's a year old this month, but the first time this happened it was only a couple months old.

Anybody got any ideas about what's going on? Is this typical? It's a drag!
Thanks

Are you using on camera flash? That used to happen to me on my 450D before I got a 430EX and a battery grip that uses 2 batteries in there, never since then.

The first time it happened I was using the on-camera flash because it was all I had. But today I wasn't using any flash.

Either battery, overheating or slow card

Since you're mentioning six shots, I assume you're shooting in RAW? (for JPEG, it should be unlimited at constant 3.5 fps)

Have you checked any noise reduction/HTP? Those drastically reduce your buffer to 2 shots.

What do you do to recover from this, then? Does it "heal" by itself, or do you wait until the next day, or what?

Colors wrote in post #12055521
Since you're mentioning six shots, I assume you're shooting in RAW? (for JPEG, it should be unlimited at constant 3.5 fps)

yes, RAW

Noise reduction and HTP are disabled.

apersson850 wrote in post #12055572
What do you do to recover from this, then? Does it "heal" by itself, or do you wait until the next day, or what?

The two times that it's happened, they were big outings and so I didn't try any shooting later that same day. By the next day, everything's back to normal.

funkyfones wrote in post #12055168
Either battery, overheating or slow card

PNY 4gb class 4 SDHC. Seems like if it was the card it wouldn't work fine at first and then start acting up.