The speedo problem sounds like your paddle-wheel might have a bad ground or loose wiring? I'd start on that side of things. Since it's perfect pass AND the gauge.... I'd think it would be the common item between them.
The starting issue could be related to heat (since it sounds like it's when the engine is warm that you have the problem?) Could be a bad starter solenoid (try to jump from the solenoid -> starter with a screwdriver next time you have this problem. http://www.ehow.com/how_5762451_jump...rewdriver.html) It could also be a neutral safety switch -- does the boat start if you jiggle the throttle back and forth a few times? The gear cable could be out of adjustment. If neither of those issues pan out, I think the starter switch is the next likely culprit. Good luck! Boat ownership can suck.... welcome to the club!
Former owner of a 1987 Supra Saltare. Current owner of a Malibu 23LSV.