Mixed effects model help

I am attempting to set up a mixed effects model, with one factor fixed (put 
in as.factor, with three factor levels) and one factor random(put in 
as.factor, with four different batches) as well as the interactions.  I have 
tried the lme function, but R continues to crash everytime I run it...
My code is:
model_lme(y~a,random=~b+a:b)
Is this way off base?
I just need to get F statistics to do hypothesis testing.

Thanks,
Chris