Why is VMware ESXi 5.5 crashing?

Yes, you should update.

That is a bug which is fixed in VMware ESXi 5.5 Update 1.

PR 1073615: ESXi host experiences a purple diagnostic screen with errors for E1000PollRxRing and E1000DevRx when the rxRing buffer fills up and the max Rx ring is set to more than 2. The next Rx packet received that is handled by the second ring is NULL, causing a processing error.

I should probably also add that it's a good idea to keep your systems up to date to resolve a variety of bugs and security issues.

The quick fix for this if you can't update your ESXi host is to change the NIC adapter from e1000 (or e1000e) to the vmxnet3 driver. The default for Windows guests is the latter, but causes problems under load.