some of our boards are having random problems meaning they work for 90% percent of the time
- usually means bad design. When a board is designed correctly, it will work all the time and reliably.
What are the problems? I believe, most of the time it will be layout issue and it is not easy to fix it without a proper re-design.
For example, many times people underestimate how important is a solid GND plane .. or a proper placement ... or keeping proper space between tracks. People just try to design the cheapest boards with bad stackup and bad layout. Proper stackup will help a lot with noises, crosstalk, quality of signals .... and not following good stackup and layout that is what in many cases makes boards work 90%
Of course, this is not generic. Sometimes there really are problems in circuit itself or in component or in assembly, but when you are sure, that your circuit and components are correct and the board is still not working reliably, then the problem is usually PCB design itself.
PS: From my experience, if replacing a bead or capacitor appears to "fix" a problem, then it is usually not true. In most cases, the problem will appear again.