Hummmm. Why's that? Badly built or over powered?
I say that because this is a typical down side of overpowering. You could also be using the wrong prop? Different pitches and dyameters effect the engines trust/torque ratio which will effect the amount of right thrust needed.

As will your pull up to vertical... which unless done correctly (pretty difficult for a beginner as any pattern flyer will tell you) will give you a false inpression of which way the aeroplanes tracking. As is an of set fin, and not forgetting the horizontal balanse... if you left wing is heavier than your right and you've trimmed it out with aileron you'll be flying under a false impression that'll only show up in a vertical plane.

The enlinement of the horizontal stab is also critical ...as are elevator halfs that have exactly the same ammount of movement If it's not 100% straight in both planes and identical in motion... your pull up will be a F-up!

I'd start with the obivious:
Check the aeroplanes built 100% straight.
If the fin is off you'll need to correct this will rudder trim.

Check the stab. Straight when you measure it from the top (Wing tip to Stab tip) and parralle to the wing and 90 deg to the fin when you look down the body.If thats all 100%, check the horizontal balanse and correct as necessary.

Next your power. Is it more than specified? if it you have some options. Add more right trust but remember, anything over 3 deg' is getting negative. Change to a different prop. Change to a different engine or try flying the verticals at a lower throttle setting. You have a rudder remember, all aeroplanes will eventually start drifting left and this need to be corrected with a little right rudder stick.

If all the above is in good shape, have a look at how you're entering the verticals, the wing must be absolute paralell to the ground. If not the vertical transition will be off.

Time to experiment.