Are you using continuous rotation servos?
If so then this code
leftservo.write(110);
rightservo.write(70);
delay (17);
leftservo.write(90);
rightservo.write(90);
probably shouldn't have the last two lines which tell the motors to stop almost immediately after telling them to move.
And "delay(17)" is just 17 millisecs. I'm not sure if that serves any purpose. Try deleting it.
...R