I must admit I haven`t run without the cache for a long time. Newer pcs are fast enough to generate fresh thumbnails every time you browse a folder but still not as fast as with cacheing on. And hard disks are so much bigger these days that I set my cache to 500 or 1000 files without worryng about the space it takes up.

With cacheing off it has to generate the temporary mini version of each image in your temp folder and it is possible that if it is doing a lot of these rapidly they are tripping over each other or getting deleted before actually loaded into the thumb view