Cat has been throwing up food pretty regularly

So, I'm hoping someone can give me some advice on this. I've had my older cat for years now and he has a habit of throwing up his food. Its not consistent, sometimes its a little, sometimes a lot, sometimes immediately after eating, sometimes hours after, etc, but it's always most of his unchewed food. I thought it was a sensitive tummy but he's on the food for that and I switched him years ago, so I don't think it can be the switch. He's had a few teeth removed (infection he got shortly after I adopted him) so I was thinking maybe he just can't chew properly? I've got the sensitive stomach food, I've tried slow feeders, I try to groom him regularly in case it's hairballs but the frequency is so odd that I'm not sure if there's something else I can be doing? The vet said it was nothing to be concerned about, but I've started keeping track of when it happens and its causing me concern. Any advice would be appreciated.