Lack of routing

This has been driving me nuts, because I can't work out what the problem is

Symptoms:

Router will stop passing traffic to the WAN apparently at random. Sometimes when it's busy, sometimes when it's quiet. A reload of the router will get it back up and running. Pings to 8.8.8.8 from the router stop working - no traffic passes through out to the internet. LAN side is fine and behaves normally

Diagnosis so far

The original router was an 881 - and I thought it was falling over because of too many NAT sessions etc. It was under specced - so has been replaced with a 1921 ((C1900-UNIVERSALK9-M), Version 15.6(2)T1, REL ).

The drop in WAN connection is less often now but still happening.

We have another way to reach the router when the WAN is down. I checked earlier today and it shows the GE0/0 as being administratively and physically up even though I can't reach my next hop. (the .93 ip address)

The config (redacted) is here: https://pastebin.com/nS0dS2Kv

Public IP addresses: have had the first three octets amended - but the subnets are still valid.

Routing table (redacted) is here: https://pastebin.com/taSVKu33

So - I think it's one of four things

a. the not-managed-by-us next hop (the Juniper) is wobbling, but seeing the interface go down and come back up when we reboot the Cisco is getting it to behave.

b. There's something inherently poor in the way I've built my config that is placing undue load onto the Cisco and causing it to fail.

c. The variably subnetted (/30 and /32) element to reach my next hop is causing an issue that isn't consistent.

d. Something else

Suggestions very welcome...