The circumstances of this timeout problem:
System.Threading.ReaderWriterLock.AcquireReaderLockInternal
Would lead me to believe it's possible there is a deadlock / locking thread issue. What this is that a process is not releasing the Sql resources correctly, and is keeping a set of records in the database locked. When the next thread comes alon and tries to read that set of records, it waits for the previous thread to complete and the lock to be released, which never happens, thus it returns with a timeout.
I would search the log carefully and see if there are any other mentions of deadlocking or resource contention. I would also see if you can reliably recreate the circumstances of the timeout (ie, is it at system startup, schedule running ,etc?) and monitor the Sql Server instance directly with a sp_who and sp_who2 command. This will tell you the identifity and last-run Sql of locking/blocking threads, from which you could work backwards and identify where the problem is coming from.
If this is genuinely because of an upgrade to the DNN Version (rather than just finding it after doing an upgrade) then it's much more likely to be the introduction of new code (whether DNN core, or third party module) than just a Sql Server running out of space or memory. Also, just because a Sql Server has enough memory doesn't mean it won't get timeouts. All the memory in the world isn't going to help a poorly-constructed query. Frequent Cpu spikes should be traced back to the process causing the spikes. If it's the sql server process. then you've got a good chance of identifying the query(ies) causing the issue and tracing backwards. Once you know the Sql that causes the problem, it's pretty easy to work backwards into the code to identify where it's going wrong.