Sorry, but we are still seeing the 503s (based in Germany). On the bright side, I am no longer able to replicate this on the cli 
Maybe I did not try hard enough (a few hundret requests)?
At first I restarted nginx, then I started playing with the nameservers (originaly we where using 8.8.8.8).
Google currently resolves ocsp.int-x3.letsencrypt.org to
a771.dscq.akamai.net.
2.18.212.56
2.18.212.72
... while my local ISP gives me...
a771.dscq.akamai.net.
2.20.189.244
2.20.190.17
Tcpdump was able to get me some references for/from Akamai:
From: 2.20.190.17
Reference #102.27d4dd58.1516779467.46d5c8d
Reference #102.16d4dd58.1516779897.e7dfba
From: 2.20.189.244
Reference #102.16d4dd58.1516780015.e86d3e
Side Note 1: Browsing through the log, the times of the 503s seem to be "clustered" together.
Side Note 2: The "timeout thing" did not affect us, only 503.
Currently I am still hoping this might be a DNS issue but I seriously doubt it.