EC2 spot instance keeps rebooting

We run a number of EC2/Ubuntu 20.04 instances in our VPC, as spot instances. All but one are fine, and the last keeps rebooting on an irregular basis. It looks (from journalctl) as though it's doing a controlled (rather than kp) shutdown and then reboots. We simply cannot work out why this is happening, when other instances have no such problems. Is there a particular parameter I should be looking at?