MY VW SHARAN 1998 GOES OFF SOMETIMES WHEN I SLOW DOWN IN A TRAFFIC HOLD UP OR WHEN I AM NEGOTIATING A BEND.

That is when clutch slip happens-when the clutch is released. If the clutch disc is worn badly, it won't hold tight against the engine flywheel-it will slip, causing that shudder you feel. Have a mechanic verify this, but it sounds like you need a new clutch.

Is a US model. Is it a van? What do you mean by "Heater Matrix"?

Just some info could be filter or fuel pump. copy and paste to google search. diesel which is takes some time to crank.
https://www.youtube.com/watch?v=yRMTEAKsScE

This problem is commonly caused by a dirty automatic idle speed control valve and throttle valve but always run a OBD2 fault code as well as the cleaning procedure. Buy a can of throttle valve cleaner (do not use carburetor spray cleaner!) from NAPA or Carquest (made by CRC chemicals) and spray it into the air intake while the engine is running, use up about 1/2 the can, engine will try to stall hold the speed up, shut it down and let it soak for 30 minutes, restart and blow out the remaining fluid, shut it down and disconnect the negative battery cable for 5 Min's to reset the base idle control. If the above cleaning doesn't cure your issues you need to have the computer controlled automatic idle speed control system diagnosed and repaired by a dealer or electronic engine control repair shop.

Change your ignation

When did you last change the battery in your remote key fob. As strange as this may sound, if your battery is slightly down in power, it will not give a full signal to your alarm/imobiliser/central locking. Because you are begining to have problems with your remote central locking, this is a good indication that this could be your problem. You can get a battery anywhere, but they are only pennies from your local VW dealer.

The fuel system is switched from L.P. to petrol by use of a solenoid.
The soleniod is operated by a relay.
The relay is electrically protected by a fuse.
Check all 3 starting with the fuse and checking back.
Please let me know what you find.

your glow plug relay is going out you need to replace it.

try to change the reservoir cap

sounds like dirty injectors try a deisel cleaning treatment red x is good run ur tank down to emtey add red x before u fill up this will clean all ur injection systome hope this helps