1995 Nissan Hardbody engine trouble

It's been doing this for about 7,000 to 10,000 miles now, but this is
the worst it has been. My truck is a 1995 Nissan Hardbody with a 2.4L,
4-cylnder, manual transmission. (Just incase that helps.)
What it's doing:
In neutral and only in neutral the engine stops, uhh, working. The
engine stalls. Now the strange part is it doesn't do this all the time.
I have found no pattern including temperature and how long the engine
has been running. The most annoying part is it never does this when
other people are around. In more detail, the RPM drop signifigantly
from idle (900)(average) to anything below that. Sometimes it will not
shut all the way down, but always it always varies in rpm while idling.
Itll fun fine for a few minutes then it slows down, but picks back up.
Its making a puttering sound when the rpm drop and it definately sounds
like the engine is having trouble staying on. This also does not matter
wether the clutch is in or it is out of gear.
The spark plugs and air filter are clean.

Those pesty mechanics have screwed us twice. First time they said it
was the EGR (something like that) valve, so they replaced that for a
hefty price.Next time, most recently they said they couldnt detect
anything wrong but they found a cracked alternator bolt which they felt
like replacing. None of these worked in the long run.

PLEASE if you can help or know of anyone that can help let them read
this and any serious help will be appreciated.

Thanks you guys.
-Will

P.S. If you need any other info, feel free to contact me.