My Ransome Bobcat ZT 219 is frustrating me with a peculiar problem. On startup sometimes the engine will only run on half throttle, almost sounding as though the choke is stuck and then after 30sec. to 2 min it will magically hiccup and go full throttle. My service mechanic checked the throttle cable linkage and checked the choke but found nothing wrong. The fuel filter is fine, and I replaced the spark plugs but the problem remains. Once up to speed, the engine will remain that way until I go to start it at the next property. Then I've got a 50% chance of it not revving all the way up again. Any ideas?