Sounds like a problem I had years ago. Very odd. Took me quite some time to figure out.
I restored a 122S (with twin SU’s) which had been sitting for many years (meaning: plenty of rust in the fuel tank).
The problem: a sudden (but only partial) loss of power when going up a long uphill. Not immediately, but some way into the uphill. I eventually tracked it down to a plug of consolidated, very fine rust powder which had formed at the bottom of the jet tube of one of the carbs. The plug had a hole in it formed by the metering needle when at rest. I assume the plug had broken loose at some stage and was able to move up and down the tube. Under normal driving conditions the plug was sitting at the bottom of the tube and enough fuel could get past. With increased flow, however, the plug moved up the tube, blocked the flow, and effectively put the one carb out of action. I discovered this by having the carbs on my workbench and blowing into the end of the flexible tube that is normally connected to the fuel bowl. Blowing gently, air was going through. Blowing harder, however, suddenly blocked the flow.
Cleaned out the jet tubes. End of problem.
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