Unable to ping host until ARP is cleared yet MAC does not change

I have an active/standby firewall configuration that has been up and running for a number of years with no problem. Since today I am getting notifications that the standby is not available through our monitoring tools. Basically, this pings all the firewalls and sends alerts. The issue I appear to have is that I am unable to see anything on the outside interface until I clear arp cache, but then nothing appears to change;

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asasov# show arp

outside 45.75.196.34 f4cc.55ac.892a 309

outside 45.75.196.35 f4cc.55ac.86ea 376

outside 45.75.196.33 0000.5e00.0106 2523

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asasov# ping outside 45.75.196.34

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 45.75.196.34, timeout is 2 seconds:

?????

Success rate is 0 percent (0/5)

asasov# ping outside 45.75.196.35

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 45.75.196.35, timeout is 2 seconds:

?????

Success rate is 0 percent (0/5)

asasov# ping outside 45.75.196.33

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 45.75.196.33, timeout is 2 seconds:

?????

Success rate is 0 percent (0/5)

asasov# clear arp

asasov# ping outside 45.75.196.33

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 45.75.196.33, timeout is 2 seconds:

?!!!!

Success rate is 80 percent (4/5), round-trip min/avg/max = 1/1/1 ms

asasov# ping outside 45.75.196.34

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 45.75.196.34, timeout is 2 seconds:

?!!!!

Success rate is 80 percent (4/5), round-trip min/avg/max = 1/1/1 ms

asasov# ping outside 45.75.196.35

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 45.75.196.35, timeout is 2 seconds:

?!!!!

Success rate is 80 percent (4/5), round-trip min/avg/max = 1/1/1 ms

asasov# show arp

outside 45.75.196.35 f4cc.55ac.86ea 6

outside 45.75.196.34 f4cc.55ac.892a 11

outside 45.75.196.33 0000.5e00.0106 18

Before I raise this as an issue with my ISP, is there anything else I can check from my end. Nothing has changed in this setup for a long, long time and certainly not today.

Thanks