oil problems...

[capie]

The mechanic who works on my bike (trained with Yamaha), as well as a top yamaha mechanic in South Africa are stumped.
If I ride at a constant speed in mid-rev range, example 100km/h (4000rpm),then stop or idle, I have oil coming thru the breather pipe into the airbox which then drains out the overflow.
The harder and longer the ride, the more oil runs out. It is not a drip, but actually runs in a little rivulet for a few seconds.

Numerous checks with the amount of oil were made. (2.6 liter)
The first job was to change the piston/rings as pressure blowthru was suspected.
No improvement to the oil leaking.

The second job was to replace the oil pump (primary and secondary) as it was suspected that the oil wasnt being pumped back up to the resevoir fast enough. This did improve the oil circulation (visibly seen thru the top when the cap was removed).
No improvement to the oil leaking.

The third job. A sieve at the bottom of the crankcase was cleaned as it was apparently blocked from when I cooked my clutch, preventing oil being pumped up to the reservoir.
Still no improvement.

Finally, we replaced an oil seal which had been inserted incorrectly and minimally damaged. This seal was behind the clutch case cover.
No improvement.

This running out of oil is not just the odd drop or two either.
Any ideas?

I forgot to add. Its a '98 XT600e (blue and white model)