The volt gauge going to 8 when the glowplugs are energized is normal. After it starts, the gauge will slowly creep up the scale, mine will stop when it's pointed directly down right in the middle of the scale. Depending on the weather and how hard it was to start, and how much I have been running the truck, it may take a while for the needle to make it to the center, but it usually will after running the engine for awhile.
The clunk sound means your starter has a loose connection or the copper washer inside it is bad.
These starters have a true "solenoid" mounted on them. When you turn the key to start, that sends juice down to the solenoid on the starter via the smaller wire. When the smaller wire gets power, it energizes the large coil in the solenoid. This pulls a plunger inside the solenoid back to the terminal end of the starter. As the plunger is going back, it's also pulling on a lever that is sending the starter gear forward into the flywheel. When the plunger gets all the way back to the terminal end, there is a large copper washer that is smashes against the two large wire terminals. This connects the large battery wire to the starter motor and makes the starter motor spin.
This copper washer is were your problem might be inside the starter. You are hearing the clunk as the solenoid moves and sends the gear into the flywheel, but there is no current getting to the starter motor through the copper washer.