John and list,
Happy New Year to all.
John asked, "When riding with much pressure on the pedals, the chain "jumps", and what I mean by jumps, is that it crosses the top of the freewheel gears. The freewheel is a Campagnolo Ergal with excellent teeth and the chain is a new Everest slotted lightweight chain."
Chain skip such as you describe is caused by a mis-match of chain and cog. Even though your freewhwheel looks like it is in good condition, it is worn enough that a chain roller is hitting the top of a cog instead of engaging behind it. This will happen most frequently on the smaller cogs of a freewheel, especially 13 or 12-tooth cogs, and even more commonly with alloy cogs instead of steel cogs. Alloy freewheels wear very quickly, and are considered special event equipment, even for professionals.
Unfortunately, there is not much you can do to correct this except to get new cogs or freewheel.
Sorry to be the bearer of bad news.
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