Not exactly, but close.

• The diagnostic process starts with looking first at symptoms and the conditions under which they occur, including any precursors.
• The next step is to look at the most likely known causes that are related to the particular symptoms and circumstances (NOT to all causes of similar symptoms).
• THEN one procedes to isolate and eliminate each of the pared-down list of causes, starting with the most likely and the easiest to check/resolve.
• If none of the "usual suspects" seem to be the problem one goes back to step one to hypothesize through observation what other causes there may be.

I'll take your initial list as an example, keeping in mind that one could probably create a much larger one for skipping under load.

• Bent chain - Does not happen on all chainwheels, would only occur when bad section of chain is at middle ring (less than every 2 crank rev's
• fd adjusted improperly - only if it's rubbing hard enough on the outer cage to shift down to the small chainwheel
• bent chainring - unlikely to cause a problem only under load and very easy to see/hear
• crud in the chainring - would not be a factor only under load
• loose chainring - Large and middle chainrings are secured by the same bolts
• loose crankarm - would not affect only the middle ring

Further, as it happens only on the middle ring it cannot be any component at the rear of the bike, as those would be factors on any chainring, and more so on the inner or outer - the middle is the easiest one for all rear cog combos to deal with.

I would take the process from the beginning but we don't have the info necessary to do so, including:

• How long "over time" did the problem progress - days, weeks, months?
• Any changes in riding habits or conditions, maintenance procedures, or any accidents/incidents prior to the problem arising?
• What rear cogs are engaged when the problem occurs?
• Does the rider spend a very large amount of riding time in the middle ring?
• What is the amount of chain wear?
• Are the middle chainwheel teeth very sharp or do they have a "shark-fin" profile?

In many cases a mechanic solves a problem through observation and logic, rather than having seen the exact problem and cause before. Those of us who were mechanics before the Internet age fixed bikes without forum, websites and often without repair manuals because we used the aforementioned "tools." Applying that do what we do know results in the following:

The problem happens only on the middle ring, therefore the most likely cause/contributor is a characteristic of that ring.
It happens only under high pressure, so another factor would be the chain, which transmits the pressure from chainwheel to the rear cogs. A third possible contribution is frame flex, which can change the chain angle, but that would seem unlikely - as noted above there are more extreme chain angles possible with the other two chainwheels. That leaves us with the chainrings and chain. Without having the full info noted above my first hypothesis would be that the chain is unshipping due to a combination of chain and middle chainwheel wear. A smaller profile on the chainwheel teeth make it harder for the chain to remain engaged when coming in at an angle, and chain wear would mean that the chain is also not engaging fully in the valley of the chainwheel. Solution would be to replace the middle ring and chain and most likely the cassette.


Of course I Googled the above....