That is my understanding of condensers Grumpy, mostly from the interchanging of the terms in documentation I have for some of my valve radio's.
Slothland, If possible could you also make sure to get some shot's of the engine and carburetor, as well as perhaps the serial number (Located on the crank case, near the carburetor, or on the opposite side to the cylinder)
Although quite a rare occurrence, I have seen issues that present like this caused by issues with the cut out, where heat causes a weakness to swell in insulation or cut out assemblies, allowing a very slight pass through, weakening the spark, or casing (more common) intermittent sparking.
It is a very rare occurrence, as the circumstances that cause it are equivalent to balancing on a pin, however, as you will have the camera out, it's not a bad idea for us to know as much as possible about the mower.
Also, How concerned are you with originality, if you want to keep the mower in original condition (It sounds like a mower from the 70's to me) then we will need to make sure all parts are replaced with correct replacements, however, if you have no concern to originality, then it may (depending on the actual case) be easier to replace the ignition with an electronic one (as bob suggested)
