Is my transmission going bad on 2007 Suzuki Reno

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car is sluggish in power and drifts back in drive on hills , check engine light go off and on, has been diagnosed to need a oxygen sensor, but that does not explain the lack of power
did change the oil gasket, and spark plugs that seem to have help for a while but now its sluggish again, but not as much as before eventually it picks up speed
(1) Answer
It sounds like you suspect the transmission but all you've told me about the car is about the engine. If the engine runs well and the transmission is slipping and not driving the car positively, then you have a transmission problem. I recommend you find a transmission shop or a Suzuki specialist to diagnose it.
Do it soon, the less damage you do the less you have to fix if you end up rebuilding it.
If you want to find a specialist, here's a directory link for you: