Do you think you were paid a "fair" salary to begin with? If you worked somewhere for 40 months and didn't get paid for 2 of those months then it's like you were paid 5% less than the agreed salary (I realize this isn't totally accurate in that the last 2 months you were probably paid more than the first 2 months). If they took you out for nice lunches and sent you to conferences then maybe they also overpaid you by 5% compared to other jobs?

I've lost out on a month's earnings before and it really sucks, but if you had a good relationship with them and learned a lot then maybe it isn't so bad.

Definitely keep up the pressure and hopefully you'll get some of the money you are owed.