Intermittent start issue

I have a 1996 Mercury Magnum Mariner 200 efi. The problem is it randomly won't start for a whole day - until the shop gets it and then it starts just fine. This has been going on for about three years. It is very sporadic. Also randomly it will bog down when I try to take off sometimes when we just get started. After an hour or so the problem goes away and she runs fine. Had the whole fuel system gone through, new filter, lines and hoses. Had to replace the stator a few years back. Last night it started but did the bog down for a while before she took off. Got me to thinking that only half the spark plugs were working. Could this be the switch box? I am pretty mechanical and can find things and test stuff but I need to know where to look. The problem is that it is so random and what ever is going on isn't fully broken yet! I have had to troll back up the trailer at a few tournaments and go home now, it is very frustrating. Any help would be appreciated as I don't want to start throwing money at it. And I will post my findings to help others once I get this thing solved!
Thank you

Scott

welcome to the BBC - please note that Mercury does not use year of motor for identification but the serial number - see sticky post http://www.bbcboards.net/mercury-mot...al-number.html and I would start here - do EVERYTHING on that list in the link http://www.bbcboards.net/mercury-3-l...tem-first.html Compression test numbers would be helpful too performed this WAY only http://www.bbcboards.net/mercury-3-l...ml#post2534330

Welcome to BBC! :beers:

Please do provide your engine serial number.

One important question: When the engine fails to start, do you HEAR the electric fuel pump running (when you turn the key ON)?