Memory Saturation FGT 1100E

Whatever the cause of memory leak is/was, I would immediately open a ticket at TAC to get it examined. But run line "diag sys top 5 40" then Shift-m to see what processes are eating up the memory, which would give you a good idea if it's caused by a bug in your version, or simply too many processes in the same type are running at the same time. 
Also check if any significant crashes in "diag debug crashlog read" when the conserve mode happened.

Toshi

What version? Is the firewall undersized?

Hi 

From the logs we note the following when the incident happened:

Memory: 16432140k total, 13148072k used (80.0%), 2805988k free (17.1%), 478080k freeable (2.9%)
Average network usage: 12712 / 77 kbps in 1 minute, 84680 / 78 kbps in 10 minutes, 70710 / 338 kbps in 30 minutes
Maximal network usage: 19746 / 115 kbps in 1 minute, 225616 / 115 kbps in 10 minutes, 225616 / 38829 kbps in 30 minutes
Average sessions: 161579 sessions in 1 minute, 162137 sessions in 10 minutes, 160841 sessions in 30 minutes
Maximal sessions: 168206 sessions in 1 minute, 171304 sessions in 10 minutes, 171304 sessions in 30 minutes

1U, 0N, 0S, 99I, 0WA, 0HI, 0SI, 0ST; 16047T, 2575F
bcm.user 165 S < 16.9 0.6 4
fltund 303 S 0.4 0.0 10
cmdbsvr 208 S 0.0 39.4 0

As for the FGT being undersized i am also wondering if it might be the case.

Could you try below commands (several times per each), then attach the output files for our references.
diagnose sys top-mem 20

diagnose sys top-all 2 20

get system performance status

If possible, when the box is under high memory condition, try to run this script and share the result to me via chat box, I will have a look.
https://community.fortinet.com/t5/FortiGate/Technical-Tip-TAC-debug-script-with-TeraTerm/ta-p/287059