engine guys, need a lil help

Got my boat last year with 340 hrs. Its at 390 now. Its a 99 mastercraft 230vrs with indmar ltr engine. (gm 5.7 liter same as malibu's monsoon 340) On the first start up when its cold it will start then die. Then i re-start it and it runs but idles rough and will die when put into gear(usually backing off trailer). If I start it and let it run for a couple min then put in gear it will stay running but will act like its going to die. After its warm putting the boat in gear has no effect on anything but does sometimes have a slight hesitation when applying throttle to take off. Last year had it checked at the local malibu dealer and they weren't able to find anything wrong. The compression is at factory specs for brand new engine, the plugs were changed, fuel pressure was checked and also is to spec, I changed the IAC valve as part of the elimination process, ran fuel system treatment through it as well. I'm not much of an engine guy but i'm capable of changing things if you guys have any suggestions. I don't know if i just have a cold blooded engine (is that even possible these days?) or an underlying problem but any input would be appreciated..