If the fan is not running up normally (probably getting noisy when running), then it may well be that the fan is failing. The fan on my Baxi Solo (different model) started failing when it was about 10 years old. The fan may be running enough to just operate the air pressure switch, which could explain the fan on light turning on and off.
If the fan is running up normally, the pressure switch may be faulty or the pipes to the pressure switch may be perished, which was another problem I had on my boiler. All I had to do was cut 10mm off the end of the pipe and push it back on to the pressure switch.
If you have the manual for your boiler there may be a fault finding flow chart in it which may point you in the right direction, but fixing it may well require a service engineer.
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