I am having a similar problem with a 92 XP I am resurrecting.... mine runs also when holding the button down and when I release it ,it seems to lose spark . I have heard that if you do let up on the button and it stops immediately, it would most likely be the starter from what a mechanic had told me. I'm a novice mechanic with these machines but have been able to get them and keep them running. If you somehow find out what's causing it let me know. I have not rectified my problem yet.