Overheating?

I took my 98 xr400 out to a local track today. The terrain was dry with areas of soft sand. The temp was 77. This is the first 4-stroke I've owned. While I was riding the bike suddenly felt like it was fouling a plug. The bike would idle and start fine, but as soon as I twisted the throttle, it would start sputtering. I guess it seemed like the choke was on, but it wasn't. I managed to get it back to the truck by riding at idle speeds. After the bike (and me) rested for about 20 mins, I started it up, and it worked fine, but it did it again. In the same spot on the track as last time. This time, I rode around on smoother sand-free areas without really opening up the throttle much, and after about 5-10 mins, the bike worked fine again when I twisted the throttle. Another guy there said that maybe it was overheating. I don't understand why it would overheat. The first time it happened, the odometer read 9 miles, and I had already stopped once earlier so it wasn't 9 miles of constant riding. Any help would be greatly appreciated.

Oh, the only engine mod that I know of is a Whit Bros. 416 kit. and I run 92 octane fuel.

Thanks,
Jeremy

Thanks for the help.

I have heard about that dipstick/thermometer. But what temp should the bike run at?

So the valves could be the problem? How hard are they to adjust? I have the service manual.

I asked him if he ran race gas in the bike. He said no, run 92 octane. I also checked the filter. I just cleaned it a few days ago, and it's still good. No problems.

There is an aftermarket headlight on the bike (acerbis). I noticed that it sits right in front of the oil cooler. Could that be part of the problem? I have a picture of it. The link is in my signature.

Anyone else have any ideas?
Thanks,
Jeremy