I'm afraid the spiders bug is a poor Skyrim engine internal timings bug. I've still experienced the same on my laptop (with vsync and iPresentInterval=1 of course). The only solution I've found was disabling HPET both in BIOS and Windows, and force CPU turbo boost, or forcing 2/4 cores. This way they fall down smoothly as expected. Notice I've NEVER had any HW issue or strange SW issues, so it must be a Skyrim engine problem...