First time on this forum so if I'm in the wrong Place I'm sorry.

My mid 70's 355 has been built pretty good. Trw 11/1 pistons, wiend dual quad intake, dual edelbrock 600's which have been rebuilt by me so I'm 60% sure they are right. Unknown cast heads, Mallory fuel pump, Mallory dual point distributor, unknown coil, crane 6.1 roller rockers. Unknown bottom end, unknown cam

Anyway this is in a boat but no marine parts to my knowledge. I inherited it from my step dad and unfortunately can't ask him questions about it. I've replaced the head gaskets and one rocker stud, I adjusted all the rockers (which may be wrong), I set the timing by ear (which may be wrong), Fuel pressure seems right, oil pressure seems right, water temp seems right

Since I did that stuff it seems to run great for about ten minutes, then begins to stutter, then dies and won't restart. (leaving me in the middle of the lake) the next day it will start on the first crank

My question is why?
Old coil?
Misadjusted valves?
Flooding?