I'm hitting some of these problems on runsaturday at the moment.
These definitely aren't being caused by machinekey errors - this is occurring on a single PC.
Some of these might be occurring due to some specific DotNetNuke behaviour - especially when the page makeup changes during a postback.
However, in one particular case - I have one specific Mac user who (regardless of whether he uses Firefox or Safari) just occasionally seems to randomly hit one of these problems).
This has me quite stumped.... but I'm still looking :)
Here's one discussion that seems to be particularly relevant - http://forums.asp.net/t/955145.aspx?PageIndex=4