Swapped out a Shimano freewheel for a Suntour New Winner and just went for my first real ride. I shifted down to my largest cog on a climb and the derailleur kept skipping back down. Never did that on the old freewheel. Doesn't do it on the trainer. Do I chalk it up to a bad day or should I make some adjustments?

The bike has Suntour Cyclone M II drive train. Friction shifters. It gets into gear, just doesn't stay there.

Thanks .