I have to manually switch on the unit. The pilot ignites immediately but the pilot does not ignite the main tank sometimes for up to half an hour and I have to manually restart the control. I have been told it could be either a control unit problem or the vent. Any advice?

It sounds as if the power vent is working okay or pilot Ignitor would not try to come on at all. My guess is there is some kind of flame sensor that isn't always sensing the pilot flame even though it is lit. Once the ignitor is turned on, the unit must see the pilot flame within a set time or it won't energize the main gas valve. Sometimes an ignitor burns out, but since it does work sometimes I doubt that it is an issue. The controller is probably doing what it was designed to do. This is a safety feature to inhibit the main gas valve from turning on without a pilot already burning. After several attempts at starting the unit locks itself out and you have to reset. Do you have the model with the display and if you do does it give you an error code?