If you're running out of RAM because of too many concurrent replications, you can try limiting how many replications can run at the same time.
You control this from the Experimenter interface (main menu > Statistics > Experimenter):

From the Advanced tab, scale down the Max Cores count (0 doesn't really mean "use zero cores". Instead it means "use as many cores as my computer has").
You can guesstimate how many concurrent replications your computer can handle at once by doing the following:
- open Windows task manager (Ctrl-Shift-Esc)
- open FlexSim (if it isn't already)
- open and run a single instance of your model
- watch the RAM utilization of the FlexSim process throughout the course of running the simulation model to completion.
- keep note of the max RAM you saw used by the FlexSim process. Round up to the nearest Gigabyte.
- Let's say for example you need about 3GB RAM to run your model. In this case you should set the Max Cores to 2, since 2*3GB is under your 8GB threshold, but trying to run 3 instances would exceed your machine's 8GB of RAM.