I am currently using MAMP 7.0. I thought I would contribute to this conversation. what worked for me was to add "-idle-timeout 9600" to my FastCgiServer Config in the <IfModule mod_fastcgi.c> block.
e.g. FastCgiServer /Applications/MAMP/fcgi-bin/php.fcgi -idle-timeout 9600 -socket httpdFastCGI.sock