One thing I learned over the years, and rarely seen posted, is that over time, because the nozzle and body that hold the nozzle are of different metals, slowly over time you can get a small drift in the position of the nozzle relative to the fluid nozzle cap. What starts to happen is that it ends up too far back from the nozzle cap, and this allows tiny bits of paint to build up and cause the air flow to surge, as it builds up and then clears.
You can sometimes detect this by spraying water through a freshly cleaned brush and watching the pattern of the spray. It should be conical, even and centered. If it drifts to one side, or you see any uneven flow at the tip - like more on one side or the other, or the spray comes out at an angle, that is what is going on.
No worries, it's usually an easy fix. It takes a few minutes tho. Remove the cap, and gently turn the nozzle in 1/16th increments to be a bit "looser". Put the nozzle cap on, test the spray, rinse and repeat. Eventually you will get the "sweet spot" where it sprays like new. Usually somewhere between 1/4 and maybe 1 full turn for an older brush. This is one of the reasons that it is vital not to overtighten a nozzle when cleaning, and it exacerbates the problem.
If your nozzle feels too loose after this, or you get air leakage back into the paint, what I do is take a bit of nylon thread, stolen from my lady's sewing kit, and run it through some beeswax to make it sticky. I wrap it once around the base of the nozzle where it meets the body, in the tiny gap, and then hit it with a lighter for a half second. Melts the nylon and beeswax into the gap, sealing it better than factory.
Microns are precision machines, and over time metal shrinkage / oxidization / over tightening or even just too agressive cleaning can introduce small variances in spacing of components that can effect spray. All too often people buy a new nozzle or needle, or ending up tearing their hair out over what can be solved by a tiny, tiny adjustment of the nozzle seating. When they leave the factory, this is rarely any kind of issue, but wear and tear in the real world are a thing, and this one little tip seems to rarely come up.
When removing a nozzle for cleaning, remember not to screw it back in more than finger tight. They are not meant to go in super tight and the body metal is softer than the nozzle, so you can introduce a bit of drift in the final relationship between the nozzle cap and the nozzle when reassembled.
Also worth noting that if you do need to replace a nozzle, you might need to do this too. They are precision machined, but even the best machining can't compensate for unknowns, like micron level metal shinkage over time. This is why it's easiest to replace with a matched fluid head assembly with all the parts matched, but... sometimes that isn't an option. So if you get a new nozzle, some tuning in how far in you screw it, should be anticipated.
A tiny bit of nylon thread and the tiniest bit of beeswax (reg wax probably works too, but the gal would probably chop me up if I raided the decorative candle box), can work wonders in making an old brush behave better than factory.