I would also suggest upgrading EPIC firmware to 3.4.0, as this increased the memory assigned for Ignition EDGE and also resolved some dodgy handling of Ethernet communication sessioneswhen communicating to other devices. Both of these could be leading to the PR1 restarting itself to get over memory problems. There are usually clues that this is happening in both groov-EPIC logs and Ignition EDGE logs, but as Ben says, you need to be an experienced support engineer to understand what the logs are reporting.
Although there is no fixed rule, In my opinion, running Ignition Edge 8.1 on a PR1, with large projects and especially if you are running other EDGE modules, may cause you to be running pretty low on memory, which in the end can lead to resets. In this case, you should at least be considering the use of a PR2.