[JustResign]

You work for the company, not the client. The fact that they haven't been paid should be irrelevant to whether you are paid or not.

If this company is living hand-to-mouth for payroll, are they going to be around for a while, anyway?

Honestly, capping your invoices and continuing to work without being paid only enables them to be shitheels.

You need to tell them that you will resign unless you're paid, in full, immediately.

I would also consider talking to a lawyer.

(Please don't take any of this as criticism -- I've been in your place before!)