VERIFY AND TUNE FTP PRODUCTION SERVER TO NOT CRASH FOR MEMORY PROBLEMS

WHAT I HAVE TO DO: I have to upload 24h/24h ftp files from camera dvr. The images are uploaded continuously with an ftp session open all the time. Actually I work with 4 dvr (4 ftp account with more session (8/8/8/32 channels)), soon I will have to work up to 40 dvr (40 ftp account). Actually with 4 dvr I have up to 9Mb/s network traffic, 3% cpu 4 core, 0,3 load, almost 1 million images day. Everything feels good. The basic server setting seems ok to a medium professional specialist. MY PROBLEM: the server crash each some hours / days. I started with a virtual box debian proftp server and I though is a bug, I tried with an Ubuntu Vsftpd server and I had the same problem, I tried with a Centos Pure ftpd server, the same! I observed the cached and buffer memory grow up in time, and I discovered that with a cron command echo 1 > /proc/sys/vm/drop_caches I don’t solve the problem. I almost solve the problem with a cron command echo 3 > /proc/sys/vm/drop_caches but I don’t stopped completely the crash of the server. I also consider that an echo 3 command is not perfect to use regularly on a production server. WHAT I NEED: I need of an expert ftp server administrator that help me to understand if is better in my case vsftpd or pureftpd, an expert that is able to study the logs and the configuration of the existent server, that understand why it crash and that tune the server for the best production and affidability performance. I will pay after 10 days of server working without crash. WHAT I OFFER: I will pay by hours, because until now by project I received request out of market price. I will choose the best skill oriented freelance that can help me, is not mandatory a little hour price, but I will put a hour limit number for the work. I consider that a real professional that know the subject, can do quickly the work. Remember that the system is already on and work good. My HW/SH configuration: A base debian 7.9 server with virtual box 5.0.26 (not the last because of a bug with proftp if I remember), a VBox share folder where the FTP virtualbox server write data. FTP VIRTUAL BOX SERVER IN USE: 1) Centos release 7 – 2.1511.e17.centos.2.10.x86_64 Kernel 3.10.0-327.36.1.e17.x86_64 Pure-ftpd v1.0.42 [privsep] (mysql authentication) Mysql Ver 15.1 Distrib 5.5.50-MariaDB for Linux (x86_64) using readline 5.1 2) Ubuntu 15.10 wily (not 16 because of a bug with pam authentication) Vsftpd 3.0.2 (mysql authentication) Mysql Ver 14.14 Distrib 5.6.31 for debian-linux-gnu (x86_64) using EditLine wrapper 3) Debian 8 jessy Proftpd