I think there are a few tests it's worth doing before you assume the worst.
Presumably you are absolutely certain the cable is adjusted right (assuming cable not hydraulic operation)?
It would be worth draining the gearbox and overdrive oil anyway, unless you have already done that. Drain the OD by releasing the cover plate underneath. Inside that is a course mesh filter, and a row of plugs. One of those (can never remember which - see the book) when unscrewed reveals a much finer filter. This can be washed out. There are some magnetic washers too. Slightly overfill (tilt car) with good oil - see previous threads on various synthetics others have recommended.
Make sure that the OD inhibitor switch really is working, and isn't letting you engage OD in 1 or 2. That would put far too much torque on it.
If you feel very carefully with your foot in the circumstances when the slipping occurs, you can sometimes sense whether it is the clutch or something else.
I'm not sure I agree about the symptoms being more like OD slippage. In my experience ODs slip when you try and engage them, not when you put torque through a disengaged OD. But I would say clutch slip is more likely in 1 and 2 as you describe.
If it only happens in extreme circumstances you could probably live with it for a long while, unless it got worse. Don't let it go on slipping - it will overheat and rapidly burn out.