Keep your gear clean and it will go a lot longer.
At least once a week I see people in the shop looking for a jacket to replace one they bought 4 or 5 years ago. The old jacket look fine, apart from being a bit dirty, but isn't waterproof anymore. Quite a good reason to buy a new jacket.
My first question is always "How long since you last washed the jacket?" Usual answer is NEVER. The old jacket is in fact still waterproof, it's just the it can't function properly anymore due to lack of care.
There are 3 things in your sweat which are pretty crap for the functioning of a waterproof, breathable jacket - body oils, salt and amino acids.
Body oils are going to reduce the breathability of the jacket pretty quickly. Think of sticking your head into a barrel of olive oil and trying to breathe. That's what the jacket is experiencing after a few weeks of intensive use. Wash the jacket and you wash the oil out so that the pores in the membrane are open again.
Salt is why you feel the jacket leaks. And most probably around the area of your shoulder muscles. With the build up of salt from your sweat on the inside of the jacket, and the water which seeps into the outer facing material because the jacket probably hasn't been impregnated for a while, you've just created a reverse-osmosis filter. You are actually pulling water into the jacket ( and cleaning it at the same time! ) Once again, wash the jacket to rinse out the salt, but also re-impregnate the outer material material so that it once again beads and the water-damp has an open path from inside to out.
These two are short term problems but by not taking care of the short term problems we end up with an even bigger long term problem - delamination. That the layers ( inner facing, membrane and outer facing aren't connected anymore. Generally around the neck area of the jacket which can be recognised by small bubble-like structures in the material. This means that the membrane is no longer supported by the outer material and can rip, in which case your jacket is no longer waterproof. So how does this happen? The amino acids in your sweat, unless they are regularly washed out of the jacket, eat the glue layer used to laminate the layers together. Or the sealing tape which then falls off and water can seep into the jacket via the seams. Once again, wash the jacket regularly and the acids won't have a chance to destroy the jacket. I've seen this a soon as 4 years after purchase, and no, it's not warranty.
So look after your gear. Wash it regularly. With extensive use, once every 2 weeks. Just using it every now and again, wash it at least every 2 months. That way you will have a choice to buy a new jacket instead of having to.