I tow a much heavier boat than yours with my 97, the first thing anyone that tows will tell you is turn off your overdrive when towing. Its your weakest gear plus you get the hunting between gears like you describe, not good for the tranny. I run mine between 60-65 with O/D off, and the RPMs are a little high but nothing overheats either. My recommendation would be to take it easier with your speed so your tranny is happier, seems like some expensive options for boosting your speed 5-10mph.