Server force restarts itself, causes incredible

Me and 2 of my friends were playing on our server for a few hours, and it was running fine for once. It usually runs at 5-8 TPS, which is terrible, but it was running at 18 TPS for the first time ever. Even when we started it, it didn't run well, so we were overjoyed it was running well. Then, we got a message about [Server is restarting to adjust system load], and we were like "no big deal, we'll just keep playing when it restarts," but when it started again, it was back the 4 TPS. I don't understand. It was running fine, we weren't doing anything intensive, and then it just restarted and kicked us back into a borderline unplayable state. Nothing works. I can't use the door I built. Farms don't function. It takes 20 seconds to eat. It's terrible.

We've tried all of those. Nothing helps. It's just irksome that the one time it works it gets restarted to make it worse.