My 99 Ford Diesel pickup will often die at idle.Usually as I slow down for a light or to make a turn onto a street.Sometimes it will idle for a bit then just die.Not all the time but often.If I add some injector cleaner to the tank and make a fairly long drive it will usually act better for a week or so.Also seems to be much worse in wet weather.Any ideas?

Your idle air bypass motor is bad or fouled with carbon deposits. Remove it and clean it with throttle body cleaner . If that don't work replace it

Sometimes that's caused by a vacuum leak near the manifold area.

Sometimes air in the coolant will cause an idle problem. Air will keep the air temp sensor from sending proper info.
Hope this helps.

Sounds like a week fuel pump have the pressure checked at a local repair shop. Also if it has a lot of miles on it the catalytic converter could be stopped up also have the local shop check it out it is real easy to check.

Is it EFI or carbureted?

Is the check engine light on when the engine is running ?
You'll have to figure out if it is a fuel problem or electrical problem.
Have you checked fuel pressure yet ?

yes its worn a bit and this is normal on manual gearboxes ,now could be a gearchange problem inside the gearbox as the reverse gear is worn on the end of the teeth ,then it could be the driver over the driven shaft is worn so the layshaft is dragging or a selsector rod ,try slowing the idle down to around 600/700 rpm as this will make it easier as this monster 7litre engine should idle at around 500 with no problem ,Now remember that iam in europe and all our or 99% of motors are manual and at least 50% diesel and this grating in reverse is normal as thier is no syncromesh on this gear ,you sort of lift clutch up a bit to move the gear clusters inside the box then drop it in i do it as second nature without thinking about it One question would you still drive this motor if had to pay around 10 dollars a gallon for petrol and top band of road tax which in ireland is around 1500 euros a year and a bit less here in spain?

According to my Haynes Diesel Engine Repair manual this is a list of possible causes.
1.Slow idle incorrectly adjusted
2.Fast idle is inoperative
3.Restricted fuel return system
4.Glow plugs turn off too soon
5.Insufficient fuel suuply to injector pump
6.Injector pump malfunction
7.Incorrect, contaminated or poor quality fuel
8.Air in fuel.
The manual is part number 10330 anywhere they sell Haynes manuals ($20). It has a very good section on the 7.3 especially if you don't want to spend money at a diesel shop every time it needs work. Hope this helps.

You seem to have the dreaded IAC ( idle air control valve) problem that Ford is famous for. The first likely suspect is the IAC valve itself, but there can be other contributing factors. With the amount of space I could not do the subject justice, but there are many online Ford forums that address this problem. I think the Ford price for the IAC is around $100. The Autozone cheapie is around $50. I think the Ford unit is a better part, but you have to make your decision. Do some reading up on the subject before buying any parts though.
Good luck.

try replacing the MAP sensor, if you have fuel injection. Your air/fuel mixture is bad.

Have you checked out the fuel pump? Check the fuel pump because Ford did put out bad pumps. My 99 did a similar problem of stalling and or shutting off when on inclines or declines. Turned out that 3 out of the 4 welds broke on the fuel pump in the tank and caused the rubber line to pinch and starve the motor of gas.