Many people use the word "sandbox" to mean an artificial limit that is placed for some period of time. We learned back during the earliest Google Sandbox investigations that there is no time period attached to the phenomena that people notice with new websites - so using the word "sandbox" can be quite misleading.
This thread, Filters exist - the Sandbox doesn't. How to build Trust [webmasterworld.com], is one of the key discussions from 2006 when the idea of a Google Sandbox was being widely discussed. Even though it is six years old now, it is valuable food for thought even now.
When the Sandbox discussion first exploded around the SEO community, Google's Matt Cutts gave a denial that any such thing had been introduced at Google. And indeed, some new sites at the time began ranking rather quickly where others took months. Eventually, Matt did identify a complex of algo factors that could look like a Sandbox - but he still was quite firm that there was no sandbox that kept sites from ranking for any specific time.
18 months is well beyond anything that has been reported even as a "sandbox" effect. I agree with the above comments -- you don't really have anything to escape from, you have something to build.