Random app crashes in the guest

[rojer]

Host: Ubuntu 10.04 (Lucid) x86_64, 4.1.4 Guest: Ubuntu 11.04 (Natty) x86

Apps in guest crash randomly. The most susceptible seems to be Adobe Flash plugin, within Chrome or Firefox. Chrome itself crashes sometimes too, but Flash is really unstable: it rarely survives 1 minute of playing a YouTube video.

What I've tried and eliminated so far:

1) Host is rock solid, no random crashes, plays YouTube video fine.
2) Memory is fine - I've ran memtest86 on both host and guest.
3) Tried downgrading to 4.0.12 - same thing.
4) Presence of guest utils make no difference - crashes happen with and without them.
5) Tried 1 and 2 CPUs for the guest, different emulated hardware (PIIX/ICH), different amounts of memory (4, 3, 2G or RAM for guest, host has 8G).
6) nothing else is happening on the host. Log in, start chrome, open a youtube video -> boom, flash plugin crashes.
7) Guest kernel seems stable, no panics.
8) On two occasions Thunderbird reported strange errors: incorrect MAC on SSL packets. This suggests that memory corruption is happening somewhere.