Transmission Help please. PCM 40A

(90’ Conbrio/PCM 40A tranny) Last summer (18’) I noticed my trans fluid getting milky. I swapped fluid, and after a few hours of running, I started noticing some slipping, and the tranny got hot. I figured that I’d done serious damage trying to get it back to the dock, so I pulled the tranny, ordered a clutch/seal kit, and had a boat mechanic do the rebuild for me. Most of the old clutch plates were in pretty bad condition. I put on a new trans cooler, replaced strut bushings while I had it all apart, and realigned it all. Regarding all that, thanks for all of your information on this forum. It really helps!
So, I tested it on the trailer…ran it through the gears for 20 minutes, and all seemed well. I extracted that fluid and replaced with Valvoline Dexron III/Mercon, which appeared to be thinner than what I’d ran before, but the trans calls for Dexron III.
So summer rolls around, and we’re all ready. FYI, I’ve always had trouble with this boat “finding neutral” to start. (i.e. I’ll have to wiggle the shifter a bit) It seemed to be worse after the rebuild. I don’t know if that’s related to my problem. Anyhow…I putted around for a bit…10-15 mph…& no slipping. I did notice that when I shifted to forward, it’d take a couple of seconds to engage. I topped off the fluid, and ran for about 5 minutes at 25 mph when I noticed slipping. I shut it down, and checked the fluid. Level was good, wasn’t hot. Reverse gear was still there, but forward had major slipping. (i.e. I’d shift to forward, throttle it a bit, and watch the shaft try to turn intermittently.)
So, we towed it in, I extracted the tranny fluid, and it didn’t appear to be discolored or burnt smelling. Now, I’m perplexed. What could be the cause of this “slipping?”…a clutch plate issue, or could it be the shifting linkage in the tranny? I’d appreciate any advice. Thanks

*I'd mistakenly posted this under the Help heading instead of Service/Repair. Michael, I did see your reply about the trans pump. I'll check into that. Thanks