Bus company First Bristol has been fined £96,250 for failing to run its services on time.
First Bristol says it is working to solve the problem
The firm appeared before the Traffic Commissioner this month to answer a string of complaints about punctuality.
Almost a quarter of buses monitored last summer were found to be late or early and three percent of scheduled services failed to arrive at all.
The commissioner imposed a £275 fine on each of the company's vehicles. The firm said it accepted the ruling.
In a statement, commissioner Philip Brown said he gave the firm credit for admitting its shortcomings and putting forward proposals to help deal with them in future.
First Bus's Bristol boss Alex Perry said: "We have made it a priority to ensure that we can demonstrate improved levels of service in Bristol.
"There is never any excuse for early running buses and we are working hard to ensure our staff observe all timing points along their routes, and we will use disciplinary action if necessary."
Earlier this month First Bristol announced it planned to pay for a full-time police community support officer to stop other motorists abusing bus lanes.