Have you tried previewing SERPs via Adwords -> Tools and Analysis -> Ad Preview and Diagnosis ?

You can select the location and tld without using a proxy.
Do you get the same fluctuations when using the preview tool?

Does this proxy support cookies? If it does and you don't clear your cookies and search history, Google will still try to personalise the results.

Without seeing your results, I'd guess that this is something to do with personalisation unless we're talking about significant fluctuations.