I was wondering if having multiple sites on a server can have an impact on rankings, given some of the sites are in the same market. Some of my sites are seeing fluctuations in rankings on a daily basis. Some terms are competitive whereas others aren't. I'll be on page 1 for a keyterm, 30 minutes later i'm on page 6, next day I'm back on page 1, next day after that page 5. This is happening for many of my keywords on numerous sites on the same server. Any ideas?
No, then shared hosting plans wouldn't be successful in search results if this question of yours was true. Make sure to never inter-link websites that you own; especially if they are on the same server. However, if you use the nofollow attribute that is fine then for inter-linking your websites.