I've been having the spinning wheel problem on my Macbook Intel Core 2 Duo (July-2007) recently; the spinning wheel appears randomly. There isn't any correlation with the size and type of applications running. When this recently happened, I opened up the Activity monitor to see if I am running out of memory, but the Free memory was around 300 MB (out of 1GB), although sometimes the spinning wheel appears even when the Free Memory is around 20-30MB. I have noticed that the current application that I am working on (be it Safari or Microsoft Word for Mac or FireFox or even Preview) shows up as "not responding" in the Activity monitor when the I get the spinning wheel. Since I noticed that my Macbook was running pretty hot, I tried the Smart Fan control app. It seemed to correct the problem but not entirely (I feel the root cause is elsewhere). The strange thing is that on some days the Macbook works just fine (statistically, I use the same applications everyday), but on some days, the spinning wheel appears every 5 minutes (very frustrating).

I have even reformatted and reinstalled my Macbook recently.

Interestingly this started to appear after I replaced my Macbook base through Apple support. Although I asked them to replace only the base (which had cracked along the edges for no good reason that I can think of), they mentioned replacing something else (heat sink...or something to that effect). Unfortunately, this problem started to harass me only after about 90 days...:-( or I could have gone back to Apple support (I do not have a warranty anymore)..

Any help, pointers greatly appreciated!