Have you tried using
Safari Enhancer. One of its options is to deactivate Safari's cache.
If it works for you, please make a donation to the developer. I find SE a very useful application.
Note too the developer's message regarding the deactivate cache feature:
"As of Enhancer 3.0, the "empty cache..." option in Safari is no longer greyed out when the cache is deactivated. I did this in order to allow people to deactivate the cache on a per-user basis (changing the menu in Safari affects the whole system). The cache is still disabled, which you can check by looking for items in the /Users/currentuser/Library/Caches/Safari/ directory. "