This may be a long shot but I will use it to second the idea of having the starter tested. Had a similar problem with a starter on a car (old VW Bus). Starter became slow, and eventually would fail to turn at all when hot. Turns out the armature was expanding due to heat and would literally lock up against the starter's cylinder housing. When cold, no problem.
"This one goes to eleven." 2007 ProStar 197 TT w/MCX and Powerslot.