I had very similar sounding issues to the OP and thought it's probably something to do with a suboptimal SD card and/or overheating or memory. I also ended up using NextDNS and tbf really like it. Does a great job at a decent price and the admin is useful. I keep thinking I should just setup another pihole but NextDNS are at a very, very sweet price point that by the time I renew (tbf I think I fit within the free tier but wanted to support it), I just throw them another 20eur.