1996 200 hp mercury efi mystery problem

This motor was rebuilt about a year ago and has had a problem with intermittent shut down. It starts fine and sometimes runs all day perfect. However quite often once a day it will shut down for a few minutes and then take off with no more problems that day. The engine will not die all together and will idle fine and rev up to the max with no load but put it in gear and will not take off and then suddenly away it goes. I have changed filters (water filter and the dry mercury engine filter). The mechanic says everything checks out fine according to meters. It doesn't make a difference which fuel tank used. Any ideas what is causing this??? Thanks, Ken

I'm no expert but sounds electrical... power pack? I don't know, someone chime in who knows what they're talking about

Repair, clean or replace all connections on your cranking battery. Do the same on all grounds on the power head. Then spread the pins on the ECU connector.