It's been a while since I worked for a car air conditioning parts company but here goes ...
As it works to begin with then I would say the system is ok with gas etc.
Possible causes are -
Sticky interior thermostat not registering temperature correctly. If you have climate control try setting it to the very lowest or 'Always on' setting.
Failing compressor clutch. The clutch on the A/C compressor may be failing to engage properly.
You need to take it to a specialist who knows what they're doing. Suggesting changing the pollen filter does not suggest someone who knows what they're doing
What you can do yourself - Start the car from cold with the bonnet open and the climate control off. Locate the compressor (follow the fan belt
) and have someone switch on the climate control. You should see and hear the compressor clutch engage. Keep watching to see if it stays on or if it slips.
My moneys on the interior thermostat being a bit flaky ....
Hope this helps.
BTW>> There's a lot of 'Experts' in air con where 'con' is the right word
Most simply 'top up' the gas with what ever gas they have. The gas/oil mix is critical for long term health of air con, don't be tempted to skip on this as the cost of a replacement compressor is £££££
Each type of gas,which used to be R12 (old) and R22? and various others each need the specific oil to go with it. The oil is carried with the gas in suspension which lubricates the seals, and critically, the compressor. Even air in the system (which is why it should be vacuumed before refilling) carries moisture which is bad ....