Nissan Repair: runs smooth, then runs, stalls often, proper delivery, heavy things

Question
I have a 1986 300zx 6cyl non turbo.  I was hauling some heavy things over a summer, and afterwards, the engine began stalling, hesitating and a rough idle, hard to start.  However, periodically, the car runs fine, starts well, silent idle and runs as before.  Had it tuned up with new wires, dist. cap, spark plugs.  Had it looked at, and they found two plugs with intermittent spark.  Changed the plugs, fuel filter and had the fuel system cleaned out twice.  What is bothering me is that it does still run perfect for a month or so, then the problem returns for a couple of months.  Could something be going out?  

Answer
Hi Henry -

I can think of 2 things right of the bat -

The ignition coil located in the distributor. This usually goes out after the engine warms up a bit, causing the issues you explain.\

Have you checked the fuel pump for proper delivery and pressure?

Aloha
calvin