Take the cranks out and wipe everything down including the spindle & splines (inside the crank arms as well)
Double check that you have the internal spacer in your BB as well while they're out. Now you can start putting everything back together and make sure to use grease on the spindle splines and bolts.
Also make sure your sprocket is installed correctly, it should be slid onto the spindle without tightening the sprocket bolt, then AFTER you snug the crank bolts up you can tighten the sprocket bolt. Also make sure the correct sprocket adapter is in there as well. And just for good measure, make sure your pedals are tight.
Do all that and see if it fixes the problem, if not, it could be something else on your bike like the chain, driver, bb, etc.