Different Google results on different computers?

Google results seem to depend on which computer and which browser is used.

[gniewko]

I'm aware that the position of my website in Google results will usually dance up and down a little bit. But recently Google is going crazy. If I search from one PC, my website shows up at #7. From another PC - in the same office and at the same time - it shows up at #5 using IE, but it's at #4 using Firefox.

This is a big pain, because I have no way of knowing whether my SEO efforts are doing any good. Has anyone else experienced this? I would understand if things varied a bit over the course of the day, but the same search, at the same time, on different PCs gives a different result. The two PCs are connected to the same ISP, so it's not a geo/network issue...