yeah exactly if you are still holding the previous SERP position then you don't have to be worry about any thing
I think there may be some dropping process happened behind your knowledge, It may be you got this position with rapid backlinks creation but you did not continued with that frequency. Trust me I got same type of result after withdraw some service.
Were you selling links? I got penalized before for selling links which dropped my PR from 5 to 0. After I removed all the paid links, it went back to PR4 in next update.
i'd say it's not that easy to get back your original pr since you have been penalized, therefore in some degree i think your good days to sell links to others is ending, it's time to turn to build your content in your site. good luck.
start building fresh backlinks from quality sites with minimum page rank 2 and generate or get some fresh content for your website.