I had the same motor and same problem. I was sure it as fuel, replaced fuel lines and filters emptied gas and started with fresh. Nothing helped. Took t to a shop spent over a $1000. They replaced fuel pumps and it was slightly better but still died after 10-15 minutes. I did a lot of web searching and found my issue.
I don't know if your coils were ever replaced but there was a recall on them and Mercury replaced them for free for original owners.
You can get the number off the coil and do some web surfing it maybe your issue. I sold that boat a few years back but after replacing coils never had another issue. I got 3 off of Ebay for around $100 a piece. Easy to replace yourself.