I starting using an orbital about 6 months ago, and had exactly the same problems as Tom and oddly came up with exactly the same solutions. I have also been using the motorised base to provide continous agitation, but the little red pegs caused uneven dev and every so often the film would "jump" over them, so I replaced them with thinner longer plastic poles that touch the lid when in place which seems to have cured both problems!!

I use about 200ml for 4 5x4 negs, and don't have a spilling problem.