it may be one of the following:
- (more likely) heat buildup in cpu or video card, caused by dirt clods & crud in the CPU/video card cooler fins or pet hair in the cpu fan of a laptop. don't keep your desktop on the floor where the pet hair and dirt is! see article clean all the fans & fins and vents. maybe you can keep the computer on a table.
- a network program is trying to connect to the network but is failing (and possibly doing it wrong), so it times out for 30 minutes and locks up the whole machine during that time frame. then it repeats the process. :-(
- you have a modern laptop (especially a Dell) and you aren't using a high-CFM fan-based laptop cooler.
- ps/2 mouse or keyboard that's shorted (gone dead) or pins bent
- a bug in the program. this can happen! causes are a while loop where the condition is somehow always true,or infinite recursion. by the way, if it's also consuming more and more memory like crazy (shown in task manager) until the machine is out of memory, thrashing the disk all the while possibly during that time trying to get virutal memory, this would be an infinite recursion problem. report it to the author so they can fix it, and the conditions in which it happened, the machine you have (os type,whether you have 32 or 64-bit, service pack level, what browser if this was in a browser).
if it's a clogged laptop fan
take it to an authorized repair shop. with 40+ DIFFERENT screws to keep track of (HP laptop), placement is critical, and sometimes you can end up with extra screws if you are doing it yourself. plus the fact that the case is very hard to take apart and the parts are hard to remove to get at the cpu. it took me and someone else to get the job done, and we ended up with "extra" screws. tell them you have a pet hair problem.
no laptop cooler
today's laptops, even older laptops circa 2004 require a fan-based laptop cooler pad, and I always recommend a high-CFM model. I consider this one of the REQUIRED pieces of equipment for a laptop (besides a USB hub, to protect your ports from physical damage).
computers don't like it hot. turn off your computer if it's not "room temp". 75 degrees is too hot. keep your computer out of the sun if you can. it is said that "if it's comfortable for you, it's comfortable for your computer", but I counter that - that can be true, but some people like it hot. computers actually like it cool.