I think that the words are very similar, especially as adjectives. But one significant difference that I perceive (as a math+linguistics major with interests in AI and computational linguistics, I do have a bit of experience with these terms) involves their derivation. I analyze computing as deriving from the verb compute, and so it inherits a notion of action or event. In contrast, computation I do not associate with actions or events; a computation can be an entirely static, abstract object. I would not say, e.g.,
1) Mathematicians in computability theory study computing.
but would say instead
2) Mathematicians in computability theory study computation.
The difference is that computing is what something does and has a more real-world feel compared to computation, which has a more theoretical feel. I think computing is also more ambiguous due to vaguer meanings relating to things that people and machines do.
If you check, computing machine and computation cost win, as I expected, but some of the others surprised me (e.g., computing capacity). I think it's sometimes a matter of a particular technical phrase becoming fixed in a field, which can happen because of a single seminal paper or such, e.g., computational complexity, for which at least one of the others would work just as well. So I think there is some arbitrariness in the statistics. That is, some of these phrases might not be spontaneously composed by each speaker for each utterance but rather are recalled as a single unit. When a phrase becomes fixed, its meaning and usage patterns can start to diverge from the meanings and usage patterns of its component parts as the language evolves. So this might explain some conceptual discrepancies.
It also occurs to me that computation has more competition from computational than computing does when used as an adjective (since the meanings of the first two are closer), so this might bring down the numbers for computation even when it conceptually fits the context better, as in computational structure, which indeed beats both of the others, with computation structure far last. Computational speed even wins, which surprises me. But I generally hear computational more often than the other two as an adjective.
As for your specific concerns about sounding weird, I wouldn't worry much. None of the options sound completely wrong. As general guidelines to avoid the oddest cases, I suggest using computing when the phrase you're modifying is closely related to a process and using computation(al) when the phrase you're modifying is closely related to a static structure or mathematical concept.