I think that this phrasing would work best, in this particular context. The idea here is that although the numbers are fluctuating, the overall pattern is on the upswing.

sugg

logical/natural comings and goings

Diplomatic relations are often described in these rather meteorological terms - see for example:

All-weather partnership: A New Dynamism in EU-Asean ties after ...24 Feb 2003 ... supportive economic policy towards Latin America and Africa then ... Asean-EU relations will continue to blow hot and cold influenced by ...
www.aseansec.org/14249.htm - Cached - Similar

BBC NEWS | Asia-Pacific | China-US relations blow hot and cold19 Apr 2006 ... As China's President Hu Jintao begins a visit to the US, Daniel Griffiths gauges attitudes to the US on the streets of Beijing.
news.bbc.co.uk/2/hi/asia-pacific/4922162.stm - Cached - Similar

And I've found in the past that 'inevitable' works quite well for 'lógico' - the sense can be very close in certain contexts.

It's not the most elegant expression, but I think it does the job here.

suggested translation:

"Migration from Ecuador to Spain has also had its impact on political relations between the two states, which have been greatly intensified through the diplomatic negotiations that this migration has entailed". (diplomatic negotiations by Ecuador to agree that Spain will accept its migrants)

I don't think "vaivenes" can be interpreted as "vicissitudes" because the author writes "dentro de los lógicos vaivenes", and in the case of "vicissitudes", I would have expected him to write "a pesar de los lógicos vaivenes".

I think it's important to include the meaning of "lógicos" as well - that is, that these ups and downs are natural.

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Note added at 4 days (2009-06-22 04:23:15 GMT) Post-grading
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No worries, Kathryn--I'm glad I could help!