My new 240z has the same problem, they tend to work when the engine is not running, but when the engine is running they either stop or keep going for just a little while.
I think there could be wiring issues aswell, the pissers go when you first turn the wiper knob.
I remember my 260Z 2+2 had reasonable wipers, I am going to try the complete setup out of this and see if there is any improvment. grease on all moving parts too.
Also I have read on hybridz or classiczcar that the use of a Honda Civic motor works well with minor modifications, there is a how to on it somewhere. including wiring diagram etc.