Telly, I was multi tasking on my reply when your post 5 arrived. The bike shouldn't be unstable and a rigid fork that's too short will only amplify the stability problem. My guess is that the suspension fork isn't sprung for your weight. Can the fork's travel be locked into one position? Are there higher rated springs or are there preload shims available (if no preload adjustment available)? Preload shims can be made from inexpensive PVC pipe if need be.
Brad