Getting "Invalid Typeref token" error at random times

I have a .Net web application that has been in operation for about 2 years. We are monitoring it with NewRelic and at least once per day, or sometimes several times per day, we get periods of time where every request results in the following error:

System.BadImageFormatException ... Invalid Typeref token.

If we recycle the App Pool, the error goes away. If we do nothing, the error goes away on it's own, and everything is fine.

I can't find a pattern of when it happens or which page causes it.

Since recycling the App Pool seems to help, it leads me to believe it's a memory error, so I've been trying to reduce my use of ViewState, particularly on large GridViews. So far, it has not helped.

Other suggestions for this problem?