The symptoms sound indicative of what we would classify as an unreliable test; meaning the test does not produce the same result when executed against the same exact product code. UI automated tests are notorious for being unreliable for a variety of reasons.
Assuming no change in the code being tested and no variability in the test (especially with UI automation) I would first suspect synchronization / race conditions between your test code and the product behavior at runtime (e.g. your code is expecting the machine to be in a particular state and the app response is slower than expected). It is also possible that your test is waiting on a particular state, or response from the system/service and the test times out before the system is ready/responds. These are perhaps the most common issues with unreliable automated tests, but certainly not the only ones.
But, an unreliable test could also point to an issue in the product (performance, inconsistent behavior, etc).
In my team when we come across unreliable tests we open a bug against the test and debug the test to identify the cause of the issue and address it appropriately. On a few rare occasions we changed the bug from test code defect to product code defect.