How much RAM do you actually have though? Regardless of how big your hard drive is (250GB) and how much free space it has, it sounds like you are running out of RAM. While the system can and will use hard drive space for virtual memory, not everything can be swapped out to virtual memory. Every program has core code that must reside in RAM only, so even if you have lots of swap space, the program is having problems because acutal running code is memory limited.
Open activity monitor (in your utilities folder) and look at how your memory is being used - http://support.apple.com/kb/HT1342
Look at the Free and Wired portions. In the example in that Kb article, the system has 2GB of actual RAM installed. If the Free section was very small, and the Wired was taking up nearly all of that space, then the system would be running out of RAM for active processes. The only solution then is to install more RAM.