Site-Site VPN, up and running but packet drops under load

WebWorks

I have aome site-site vpns running, and also l2tp vpn to the sites.
Basically all running without problems, except that the clients complain about "performance" issues on the site-site vpn.
Monitoring now showed reuccuring packet drops, under load, in the site-site vpn. 
L2TP is not affected.
I tryed many things, read logs, adjusted default session per host, mss to 1360, disabled "Replay Detection" and so on.
Problem persists and there is no hint that the tunnels are rebuilt, just "packet drops".
Any idea where else to look for?

mMontana

Could you please share more info about your tunnels?

L2TP AFAIK use 3DES/MD5, the other ones?

lalaland

Can you take a screenshot about packets drops log in device?
Maybe we can see that if it drops by ADP, UTM, or something else.