Bit of a funny one. When i reverse, or slowly pull away (like parking, three point turn etc) my RPM drops fairly low, so low that it has just cut out a few times, and the oil light has flickered on and off. When you are moving along its completely fine.
Mine did exactly the same for ages. When it was put back together and remapped we increased the idle slightly n it doesn't do it. Mine is a 2.5. It could be a faulty oil pressure sensor. I wouldn't worry bout it too much. If it does it when drivin then I would worry.
... simples...