Is it doing it when going into both forward or reverse? If the shift linkage is slightly out of adjustment, it can present similar symptons, which may be more apparent when cold. When mine was slightly out, it would take several seconds to engage the forward gear. If it only does it in one direction only (F or R), it could need some adjustment. Different boat/transmission I realize, but something to check.
1988 Tristar 190 Sport, 351W w/GT-40's, 1:1 Velvetdrive