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I agree that the most likely culprit is a lean mixture but there are many ways for that to occur and chasing down the cause is often a matter of trial and error.
A number of years ago I experienced the same problem and after a lot of dead-ends finally found an object in the fuel tank that was intermittently getting drawn against the fuel pick up to cause fuel starvation and then when I let off the throttle it would fall away to the bottom of the tank--only to happen over and over. I finally pulled off the fuel sender, examined the tank's innards and found the cause--the almost skeletal remains of a large leaf that somehow got through the filler cap (don't even ask how that could happen, much less with a 100 where the filler is accessible only when the boot lid is open!!)
Removed leaf--solved problem.
A number of years ago I experienced the same problem and after a lot of dead-ends finally found an object in the fuel tank that was intermittently getting drawn against the fuel pick up to cause fuel starvation and then when I let off the throttle it would fall away to the bottom of the tank--only to happen over and over. I finally pulled off the fuel sender, examined the tank's innards and found the cause--the almost skeletal remains of a large leaf that somehow got through the filler cap (don't even ask how that could happen, much less with a 100 where the filler is accessible only when the boot lid is open!!)
Removed leaf--solved problem.
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