rough cold idle start when not use for a week or so

[ynot]

96 port side big block Mercruiser with injection
the engine start right away but sluggish idle until it reach 160 temperature then run smooth
couldn't figure it out
if I restart the engine it great and keep running smooth
I re start the next morning the same result runs good
when I re start a week or two later then issue comeback again sluggish
like to hear any comments or advise
thank you