Another thing worth looking at is whether the tyres have worn unevenly or are under the required pressure, both of which could cause the "pulling" effect you mention.
If not, check when you're riding that you're not gripping the bars too tightly or leaning forward and using them to support your weight. Squeeze the tank between your thighs (ooo-err missus!) and let your arms relax so your elbows and shoulders drop down and relax your grip to the bare minimum you need.
One other thing you might try on a long, straight empty road is to accelerate up to a reasonable speed, then pull in the clutch and block-shift down to neutral and (carefully!) let go of the bars and see if it starts to drift.
If not, check when you're riding that you're not gripping the bars too tightly or leaning forward and using them to support your weight. Squeeze the tank between your thighs (ooo-err missus!) and let your arms relax so your elbows and shoulders drop down and relax your grip to the bare minimum you need.
One other thing you might try on a long, straight empty road is to accelerate up to a reasonable speed, then pull in the clutch and block-shift down to neutral and (carefully!) let go of the bars and see if it starts to drift.