Connectivity that works, then dies for a few seconds, then works again is a classic duplicate IP signature. When two hosts claim the same address, switches keep relearning which MAC owns it, and ARP flaps between them — so packets land on the wrong machine half the time.
Confirm the conflict
dmesg | grep -i 'duplicate address' # "IPv4 duplicate address 10.0.1.50 detected (also claimed by aa:bb:cc:...)" arping -D -I eth0 10.0.1.50 # -D = duplicate-address detect; a reply = conflict
arping -D is the key tool: it asks "does anyone else already answer for this address?" A response means yes, you've got a conflict — note the MAC so you can track down the other machine.
Fix it
Give one host a unique address. If it's statically configured, fix the config (e.g. /etc/network/interfaces) and reapply; if it should be dynamic, hand it back to DHCP. The usual root cause is a static IP that overlaps the DHCP pool — carve static assignments out of the dynamic range so this can't recur.
Practise it
IP conflicts are one of the simulator's networking incidents. Drill them in Linux troubleshooting practice.