I have an old bike to get me from the station to work and back at the far end of my commute. It was never up to much, but anything good would be stolen in no time as well as degrading from being stored outside in all weathers.
It's got 3x6 gearing, a freewheel system. Recently, when I shift into the big ring while accelerating reasonably hard I sometimes feel a lack of resistance to the pedals, for 2 or 3 pedal strokes. It's not the rough, sudden feeling of a chain skipping, and besides only happens just after shifting up. It doesn't feel like it's tried and failed to shift up, because there's less resistance than in the middle ring, and also that tends to catch and release. It feels like something is slipping - but what? Unfortunately I haven't been able to replicate it at times when I can look down and back to see if the rear derailleur is doing something odd. It's happened more often when I'm already in the smallest sprocket, but today I deliberately shifted up while in a middling sprocket.
It's not slipping as freely as when my freehub on another bike packed up, and in that case it was equally likely in any gear.
I'm not very familiar with freewheel systems, and that might be making me inclined to blame the unknown. I've had a few but whenever I've tried to work on them they've been rusted and impossible to remove - that's why this back wheel came from another cheap old bike.