Why do chains slip?

[Smurfy]

Thank you @YellowTim. My bike is a cheap Cinelli. It has a Miche Reflex rear wheel and a Campagnolo cassette. I would be delighted to splurge a few quid on a dogs' danglies freehub if it solves my fear of ending up on the tarmac, or gasping on the top tube. I'm still a bit cautious about getting out of the saddle. You would be too if you went through what I did, but I'm grateful for your advice and suggestions.

I guess I just need a team car and a bunch of mechanics.

Some of the stuff I've owned.

Hope freehub: Never slipped
Shimano Deore DX freehub: Never slipped

Shimano 6 speed freewheel: Slipped occasionally for a few months, but grew out of it and worked OK until retired due to sprocket wear.
Generic copy of shimano freehub: Currently on my commuter, slips once or twice a week, but not badly enough to cause an off.