A few things to check.

Concentricity of chainring and rear sprocket. When you turn the cranks the center of the lower loop shouldn't rise and fall more than 1/8". Also there still must be a bit of slack, at the tightest point.

As stated above there may be a stiff or otherwise damaged link, but it's hard to spot on a single speed. Push your rear wheel forward so there's significant slack in the chain. Pedal slowly both forward and back, looking of any link that doesn't engage, or disengage smoothly. If you find it it could be the chain, or the sprocket.

If all else fails, use a dry marker to color the tips of all the chainring's teeth. Tied the bike until you feel the problem a few times. Take off the chain and look for uneven wear of the black of the chainring teeth. If so, these are high, or possibly burred, and need th dressed with a file, or the chainring replaced.