It seems funny that you feel over the front with the 90 but the steering wanders. I ran both a 90mm and a 60mm on a 2005 Five with Pikes and big wide bars (up to 711mm on occasion, but I have good reasons for that honest). The 90mm was better. That said I only had 10mm of spacer stack.
Is the frame the right size? If 90mm corners OK but you feel you're too far forward and 70mm leave the handling a little twitchy, it could be you need a smaller frame size.
Or you could try an 80mm (On One had them at a tenner recently)
And looking at the photo there might be room to roll the bars back a little and put the 90 back on – but that's a real personal taste one.
Of course there is merit to Jedi's comment.
And drop the saddle an inch – it's not a carbon fibre Euro hard tail racer.