which de-noiser effect are you using? if its causing a side effect, then it would be better to use another de-noiser, rather than try to fix a problem it creates.
premiere has no real slow motion effect. it will either duplicate frames or frame blend. neither are good options for todays fast refresh rate monitors. if you still want to use frame blend, you can select the slow-mo clips in the timeline and right click to turn on frame blend just for those. if you have access to after effects you can try the one in there, or look at third party plugins like twixtor. if you shot high fps like 60, you can then slow mo that footage and premiere will have frames to work with, and wont need frame blend.