BI Precalc workbooks hang (open excel session)

We are running:

SAP GUI 7.40 Patch 15

Excel 2013 32-Bit

Windows Server 2012 R2

BI Components 7.40 Patch 900

Front end check runs fine without errors.

3 Precalc servers are configured to run and these all start and connect without issue.

The Problem:

Intermittently, workbooks will hang on the precalc server, only to time out. I have run the system in console mode and it appears that excel opens a session, then opens the workbook in another window. When the precalculation completes, it closes the second window but not the initial excel window. As a result, the workbook sits in the "Currently processing queue" (in RSPRECADMIN) until it hits the timeout value. I have been through every note and article I can find but it seems that no one else is having this issue. I am open to any and all suggestions for a possible resolution.

Errors:

In the logs, I only see the following error:

Exception: Attempted to read or write protected memory

Occasionally, a server will become unavailable and the following message will appear:

The RPC server is unavailable

Additional Info:

I also see the line: SAP GUI 720 Connection Mode

I would think this should be 740, but I am not certain it matters.

The results seem to be intermittent, so the configuration is generally ok, but some conditions seem to trigger the hangs.