Then it probably is back to a memory limit of some sort.
I think you said you increased the Memory Limit from 128M to 256M (under Configuration...Google Merchant Center Feeder Configuration...Memory Limit). Did that let you increase the number of products, i.e. did you test how many products you could get through at 128M and then at 256M? Did it make a difference at all?
It may also due to host limitations. If you look under Tools...Server Information, what is your PHP Memory Limit?
Here's an example of mine:
Server Information
Server Host: xxx.xxxxxhost.com (xxx.xxx.xxx.xxx) Server OS: Linux 2.6.32-696.299.3.2.cp6.x86_64
Server Date: 07/30/2017 13:22:30 Server Up Time: 13:22:30 up 38 days, 10:27, 0 users, load average: 2.35, 2.65, 2.71
HTTP Server: Apache PHP Version: 5.6.30 (Zend: 2.6.0)
PHP File Uploads: On Upload Max Size: 64M
PHP Memory Limit: 160M POST Max Size: 64M
Database: MySQL 5.5.55-cll Database Host: localhost (127.0.0.1)
Database Date: 07/30/2017 13:22:30 Database Data Size: 48,274 kB
Database Index Size: 10,354 kB MySQL Slow Query Log Status: On
MySQL Slow Query Log File: /var/lib/mysql/xxxx-slow.log MySQL Mode: (None set)
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