If your sheet length monitor reads out the proper sheet length, then the variation is caused by web slippage at the point of nip (between the squeeze roll and the draw drum).
Check your roll stand for proper brake pressure. As a rule of thumb, the initial brake setting should be set so that a full roll can be turned by one man.
Another area of adjustment would be the squeeze roll pressure. By increasing the nip pressure, you should realize better sheet length control on start-up.
