Plumbing Question - Sinks drain slowly, what to do? I have experience next to sinks, installing, cleaning out traps, etc......

I have experience next to sinks, installing, cleaning out traps, etc... but I'm not sure about this. for over a year, our kitchen sink sporatically drains slowly, sometimes drains fine, sometimes highly slow. now a 2nd floor bathroom sink is suddenly draining severely slowly all the time. I took apart the traps and inspected them, no clogs contained by the trap or anywhere from drain to trap, so the problem must be after the trap, when removing the 2nd floor sink trap, the sewage smell was overwhelming. I've also tried forcing marine down the drain after the trap using a garden hose (a trick I learned from a plumber) and drano and snaking next to a hand powered snake, no give support to. Also we have one distinctive toilet that tends to flush (drain) slowly and sometimes get clogged, if it's any help. We own other toilets and sinks that drain perfectly fine. I'm not sure if this is an nouns lock/pressure thing but I'm looking for any warning to try before resorting to calling a plumber. Thank
Answers:    sounds close to an airlock, you will need to check your vent for blockage, or install one, you can get a sniffer vent for beneath your sink.
good luck ... sounds close to it's time for the plumber.