I have an intermittent workflow issue that's proving exceptionally difficult to troubleshoot. Given the details below, what would be the next steps in finding a cause for this issue?

**Details:**

This is SharePoint 2013. The workflow was created in Designer 2013 and is using a 2010 template. It's a fairly simple workflow that creates a new list item, creates a matching document set in two separate libraries, then updates the list item with links to the document sets. The second step (creating the document sets) is an impersonation step that uses a service account. The service account is a site collection admin and also has full write access via policy on the web app.

Every great once in a while, the workflow will fail on creating the document set. The workflow status will be "***The workflow could not create the list item. Make sure the list exists and the user has permissions to add items to the list***." The outcome is "Unknown Error" and so far I've been unable to find any related entries in the ULS logs when this happens. I can live with this once in a while, but for one particular user it's happening a LOT. I've been tracking her workflows the past few weeks and it can happen anywhere from 25% of the time to 100% of the time. In other words, she'll have some days where she creates 20 list items and 1 fails. She'll have other days where she creates 20 list items and 19 fail - it seems completely random & intermittent.

When the issue happens, deleting the list item and creating it again (to trigger the workflow) usually works. Sometimes it doesn't. This issue only seems to affect this one user. The workflow completes normally 99% of the time for myself and everyone else.

Here are the steps I've taken so far to troubleshoot:

* Made her a site collection admin. Same issue.
* Confirmed that she has write access to both the task & history lists used by the workflow.
* Put a 5-min pause between each step in the workflow. Same issue.
* Put the document set creation part in its own separate workflow. Same issue.
* Had her log in as herself on someone else's computer and try. Same issue.
* Changed the workflow to use her own credentials instead of the service account. Same issue.
* Deleted the user from the site collection and set up her permissions from scratch again, then had her try. Same issue.

Also, to give you an idea of how random this is - when I had her login on someone else's computer and try, here's how it went down:

1. Tried on her computer. Workflow failed.
2. Tried on the other computer. Workflow failed.
3. Tried a 3rd time back on her computer. Workflow completed normally.

Say what? Exact same name & field values on the list item, and I confirmed that she's doing everything exactly as I would. Yet it fails 2 out of 3 times for her.

Any suggestions are appreciated - I'm just not sure where to go with this one.

**UPDATE:** We never did find a resolution to this, even after removing her from the site collection and re-adding her. As it happens, someone else took over the role (previous user was temp) and the new person isn't experiencing the issue. So case closed, I guess.