I have also had mine for over 20 years and I had this problem a while ago. I also had the problem with blockages of the small holes (and the milk frothing pipe) and found that repeated de-scaling - 5 to 10 times over a day with time to soak eventually cleared it.
All the answers above are good and I have made sure to look after the gasket, tighten the cap and de-scale the machine at least annually.
However, today I found a new problem - as mentioned above there's a pressure relief valve built into the cap, for safety. Age seems to have taken its toll on the plastic of the cap and the plastic rim that holds the spring assembly in place has fractured and the valve has fallen out. I didn't notice this at first because the break is clean and nothing looks out of place, but I looked inside the boiler and saw the parts of the valve sitting there, which was something of a clue ! I'm currently searching online for a replacement cap, but no luck yet - so does anyone know a good website for this?