My slow cooker keeps overheating and ruining soups—what’s going wrong?

I recently got a basic slow cooker to make soups and stews for busy weeknights, but every time I try to cook for 6-8 hours on low, the soup ends up boiling aggressively instead of gently simmering. I’ve checked that the lid fits properly and I’m using the recommended amount of liquid, but the temperature still spikes way beyond what I expected. I don’t have a fancy model, just a simple one with low, high, and warm settings. I want a slow, gentle cook so the flavors meld without reducing the liquid too fast. Has anyone else had their slow cooker run way hotter than it should? Would a separate thermometer help me troubleshoot, or should I just return it and get a different brand?