I had knee surgery on friday and I have had a start and stop stream since, is this common?

No. Sometimes narcotics (pain medications) can cause urine retention (difficulty starting urination). Other problems can arise if you had a catheter placed - i.e. Urinary tract infections. Bottom line, talk to your doctor about this.
Need more info. If you mean your urine stream, it can be from a foley catheter, it can be from narcotic pain meds, it can be from dehydration. Please speak with your surgeon.