87 2WD Pick Up - Two Decades of Starting Problems

My folks gave me their 1987 Nissan 2WD pick up (Z24i engine) several years ago and ever since I've moved to the south (2005), it's been having trouble starting during the hot and humid months. It's pretty much a given that the truck won't restart if I've driven it and let it sit for a few minutes. This rarely happens during the cool winter months (but has happened during the rain). This has been happening on and off (albeit much more rarely) since the early 90s. We've owned the truck since it was brand new, keep exhaustive records on nearly everything done (its ridiculous how many notes we've got, right down to how much gas we've put in the tank and how much its cost us over the years).

Every time I take the truck in to mechanics to look over the problem, they either respond with dumbfounded looks (translation: "I dunno") or think they've found the solution and replace things. My dad and I have also replaced a few parts when we thought we had it figured out. Inevitably, however, the truck stops starting reliably within a month or three (longer if the weather's cool). Typically, when the vehicle doesn't start, I can hear the fuel pump running, but nothing else.

Here's what's been replaced: battery; alternator; fuel pump, ignition switch, clutch interlock switch, every pertinent fuse, spark plugs & cables, and distributor cap. The mechanics always say the starter is just fine.

Any ideas would be appreciated.

I forgot to mention that most of the time the truck start if jumped.

why did you not change the starter relay ???

I forgot to mention that I did also replace the starter relay.

There's a ground cable from the engine to the body that is often overlooked.
I'm guessing that when it doesn't work it doesn't crank at all. Connections at the negitive battery cable to the body can also be corroded.
This seems to be a common problem. If it's not in the parts than it has to be in the wiring that connects those parts.

I had the same problem with my 86 I just bought. I took the starter out and had it tested at autozone. It tested good. After a few days of frustration, I replaced the starter anyway and It started right up. It still makes a clunking noise once in a while when trying to start it, (which is why I wanted to replace the starter anyway) but It has always started.