Last week took boat out for a run, first run in a few weeks. As I was bringing up the rpms to get on plane, at about 2000 rpm the port engine began to rev suddenly as if I shifted to neutral. I backed off to 1000 and all seemed normal and I repeated the process three more times, each time I got to 2000 or so the port revved with no noticeable increase in thrust. Boat is an 89 300 DA with twin 5.7s and Merc Alpha one gen one drives. Port drive is counter rotation. I removed prop when we got out of the inlet to our swimming spot. It has a rubber press in hub ( Rubex). It didn't look bad and I could not get it to slip with boat in gear and me pulling hard on a blade, but I'm not a 320 hp engine either. I smelled NO rubber odor while accelerating and the drive shaft seemed to be increasing in speed when engine free revved while trying to plane. Boat never heated. I'm hoping it's a spun hub and not the coupler BTW coupler is 4 yrs old and alignment in spring was perfect. I have not hit anything with either prop. Back in spring on initial launch, the shift cable which was new, was a bit out of adjustment and that drive didn't engage forward right away and revs built up and she slammed into gear hard. I adjusted cable at dock and been fine since. I'm wondering if that shock broke the hub free? Only thing I can think of unusual that happened. Aside from hub or coupler, what else should I check? I bought a new prop and a replacement hub and will try that next time I'm down.