I had pretty much the same issue some weeks ago when I disconnected my router and moved it. It behaved the same with loss of several ping packages in a row and then 4-5 that went through.
After upgrade, and a total re-flash the issue was the same.
But I also have a router that was provided by my ISP that are connected between the internet port to the outside world and my OpenWRT router. The ISP router had the same ip adress 192.168.1.1 that my OpenWRT router had. I changed ISP router to 192.168.0.1 instead and kept the 192.168.1.1 on my OpenWRT router and it now works. Even if you don't have a extra router on the line there might be a IP collision that results in the same result. Hope this helps
The weird thing here is that I think they both had the same IP address before I disconnected them and it still worked somehow.