On the NetScaler checking the CPU stats you may find similar output as below for CPU stat command on CLI:
>stat system cpu
 CPU statistics
ID         Usage
4            100
5            100
3            100
2            100
1            100
 
In above case this NetScaler has 5 Packet CPU and the output will be shown as per the number of Packet CPU's.

Checking the newnslog counters (under /var/nslog folder) will show that there are a lot of loopback packets on the NetScaler which is higher than the traffic interfaces:
#nsconmsg -K newnslog -d current -s disptime=1 -g nic_tot_rx_packets | more
1       0        9189639189 nic_tot_rx_packets interface(10/1)
    3       0        9129888925 nic_tot_rx_packets interface(10/2)
    5       0       86622975465 nic_tot_rx_packets interface(10/3)
    7       0       86824396495 nic_tot_rx_packets interface(10/4)
    9       0        3525761411 nic_tot_rx_packets interface(0/1)
   11       0      681495258986 nic_tot_rx_packets interface(LO/1)
   13       0       18319518984 nic_tot_rx_packets interface(LA/1)
   15       0      173447330798 nic_tot_rx_packets interface(LA/2)
   17       0      195292661485 allnic_tot_rx_packets

If you do a NetScaler packet capture, you can identify that looping one particular packet is causing high CPU and most of the traffic on NetScaler is the packet as mentioned below:
NSIP -> 127.0.0.2    ICMP 105 Destination unreachable (Port unreachable)
Also you could identify that the actual traffic on the NetScaler is very low compared to mentioned looped packet.
 

This is seen when the configured nameServer returns a server failure response and the packet is looped into NetScaler.

Workaround:
Execute below command in NetScaler CLI, the command will drop such packets causing a loop within the NetScaler
>set ns rateControl -tcpThreshold 233 -icmpThreshold 100
Fix: 
The fix for the issue has been applied to 11.1 59.X, 12.0 59.X and 12.1 49.X builds, which will be released in Q3 2018. 
 

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Problem Cause

Caused by issue #707489, where ICMP error packet destined to 127.0.0.2 is looping in NetScaler and causing high CPU.

NetScaler is reporting high packet CPU continuously and performing a reboot or failover works fine for some time(for a few days) and then packet CPU uses starts increasing again and even when the actual user traffic is very low.