new owner/member power loss

Hey everbody, I just picked up a 1985 zl900 which I have not been riding too often (no licence yet) and over the past 100K it has been a bit of a disapointment. The power seems to drop a lot, at 5000 rpm it's like it goes from a four cylinder to a sewing machine. I'll pull in the clutch, rev the p*ss out of it for about 3-4 seconds and it's ok for a while. What could it be - plugs, gas line, carb?? Any ideas??? It seems to be happening more and more, but remains an intermitant problem. Otherwise, this bike, my 1st Kawa, kicks *ss. Also, does anybody have information or any ideas about the torque or horse power of this particular bike? Is it really as big and as powerful as it feels? Thanks for any input.