Have you rendered your timeline?
If you're sceneline mode, switch to timeline mode. Is there a red line above your clips? If so, press Enter and let the program render your timeline (the red line will turn green).
This should greatly improve your program's stability and performance.
Also, ensure that the disc where you are storing your video files is formatted NTFS and not FAT32 (as they are when they come from the factory). FAT32 drives have a file size limit that can choke video work.
And, if this 500 gig drive is not your C drive, ensure that your C drive also has at least 30-50 gigs of free space, freshly defragmented and with its temp files regularly cleared.
thanks for your suggestion, didn't know about timeline rendering. I tried it, but unfortunately it failed after a few minutes with the error "Error compiling movie. Out of memory".I'm really puzzled why Premiere doesn't like all the available memory I have.
I looked at the other suggestions as well. All my disks are NTFS-formatted. My C: drive has 30GB free space, I defragmented it a few days ago.
I find the tips in this ARTICLE very useful for freeing up all possible resources/memory/CPU cycles. They are first a general tune-up, and then some steps that I take before ANY editing/authoring session.