I'd say as a first step, check your sag front and rear. I don't have the links handy to any of the sites with good directions for doing this - maybe someone else can help out on this? Even though you can't adjust the front sag, you can get the back set up and at least know where you are with the front.
If you haven't played with the damping adjustment on the rear, or don't know for sure what it is set at - try setting it to stock specs and going from there after setting sag. Turn the adjustscrew on the bottom of the shock in all the way and count the clicks. Then turn counterclockwise for eight clicks and work from there.
I have a 95 M900 and weigh about 220 with gear on. I ended setting my rear preload to about 26 mm (that's preload on the spring compared to the spring's measured free length) to get sag within specs. I think I ended up with the damping screw opened a total of 11 clicks to get the best results. My front end is too soft, but so far the bike has handled well, even in the mountains. I'm changing front fork oil any day now, and am really considering throwing in some Racetech springs of the correct rate while I've got them off the bike.
I have a 96 m900 when I first got it I dropped the front so the forks were about 1" above the triple clamp, and raised the rear up about 1/2". I also got rid of the Dunlop 208's I hated them (had a 2 wheel drift without pushing hard), and put on Pirelli Diablo's, that did the trick. It steers much quicker not as quick as the 996 but pretty good. I just took it to my first track day and used up every bit of tire on the rear, it handled great, but I do have a crappy steering damper that may have made a difference.
I must say I have had enough seat time on prillers
and haven't felt the Monster was lacking in handling at all.
I'm not sure what a real bike is but you should expect
the monster to be as good as a priller....
I suspect the 208's and ten year old stock suspension.
208's and tank slappers are a familiar symptom to me.
Hi' I have a 94 spec 900 wiht the 'poverty pack' suspenders and too was horrified at how badly it handled at fun speeds.
I know it sounds too obvious but i have found that things like tyre pressures and head tension are crucial as well as the radius of both tyres as there is that angle where the bike gets off the middle bit and starts to roll better.
probably not an issue for a genuine scratcher.
Forks are bout 5mm from the top clamp but it still weavers at speed although turn in is pretty good.
hope this helps.