Hi, a 503 is usually due to the level of traffic, may be not to the cPU load and a cache does not lower the traffic, it raises the traffic.
Anyway, do not set the cache max age to zero, set it to 24 hours for example. Check the cache/hyper-cache folder to see if files are created: the signature inside a page can be misleading. Or track with firebug (for example) the browser request to the page and check for "x-hyper-cache: hit" on the response headers.