Jetlink route 797 leaves LHR Central bus station every 2 hours between 0640 and 2240 for Cambridge via Stansted.
The LHR-Cambridge route is hourly, but alternate departures from LHR go via either Luton Airport (route 787) or Stansted, giving a 2-hourly service interval to each airport. The timetable indicates the journey time to be 90 minutes.
Each route also leaves T4 about 15 minutes before the Central bus station departure time.
I've used the 787 for several years to get from Luton Airport to LHR and never missed a flight yet, although it's true the M25 can get horribly congested at virtually any time of day. The bus schedule allows for this possibility to an extent, although of course if the M25 is blocked solid then the bus will be late. The trick is to give yourself a generous connection time.