Virtual Assistant advice

I have a regular need for a small project to be done, a little bit of research, some following up on projects and so on. I've thought about using a "Virtual Assistant" for this but there is now so much noise in the area that it's hard to navigate. An example of many many is Zirtual.com

My needs are definitely episodic, so I don't want someone that I pay a fixed monthly amount to unless it's pretty small, knowing that it can go up and down depending on the work.

Any feedback on your experiences or thoughts about this would be really interesting!