Excel 2010 Powerpivot workbook constanly crashing when editting pivot tables

I have a quite large powerpivot work book that I store on dropbox.   If I want to add the same field twice I go tot  the normal pivot table field list and drop the field into the pivot table again.  This used to be fine.  I could then make it show the difference form previous period or similar.   Recently after doing a couple of these or changing the number format excel just crashes.   I have tried rewinding my workbook version back to older versions and still the same issue.  

I tried just taking a simple single sheet of data and creating a new powerpivot model. i then dropped a couple of measures on to the pivot table and then the same measures again form the regular pivot table field list. I then tried to change on of these to show values as % column total and crashed. so the problem is excel not my workbook.

I have run the repair through control panel and run sfc /scannow but problem persists. In event viewer the error shows as

Faulting application name: EXCEL.EXE, version: 14.0.7109.5000, time stamp: 0x522a4035
Faulting module name: EXCEL.EXE, version: 14.0.7109.5000, time stamp: 0x522a4035
Exception code: 0xc0000005
Fault offset: 0x00000000006d5a7b
Faulting process id: 0xabc
Faulting application start time: 0x01cf67968e9d437e
Faulting application path: C:\Program Files\Microsoft Office\Office14\EXCEL.EXE
Faulting module path: C:\Program Files\Microsoft Office\Office14\EXCEL.EXE
Report Id: 371233aa-d38a-11e3-b47e-90b11c9a7548

Windows error Reporting

Fault bucket 135635980, type 4
Event Name: APPCRASH
Response: Not available
Cab Id: 0

Problem signature:
P1: EXCEL.EXE
P2: 14.0.7109.5000
P3: 522a4035
P4: EXCEL.EXE
P5: 14.0.7109.5000
P6: 522a4035
P7: c0000005
P8: 00000000006d5a7b
P9:
P10:

Attached files:
U:\Users\Mike\AppData\Local\Temp\416382.cvr
U:\Users\Mike\Dropbox\In-n-Out\INO Data Model\INO_Data_Apr14.xlsx
U:\Users\Mike\AppData\Local\Temp\CVRC1C8.tmp.cvr
U:\Users\Mike\AppData\Local\Temp\WER6057.tmp.WERInternalMetadata.xml

These files may be available here:
U:\Users\Mike\AppData\Local\Microsoft\Windows\WER\ReportArchive\AppCrash_EXCEL.EXE_6e3f938ed926058ac63c0bf295faed7891c5_150ec792

Analysis symbol:
Rechecking for solution: 0
Report Id: 371233aa-d38a-11e3-b47e-90b11c9a7548
Report Status: 0

Really appreciate any help as its completely unpredictable and having to save my work every few minutes.

Thanks

Mike