Sudden rough starting/running is usually just one or more bad glowplugs. Does a 10,981 300TD even use glowplugs?

Read through these guides and search the forum for "glow plugs" and you'll find what you need.
http://www.pauldrayton.com/uploadfiles/merc/Service/W123/Index/617T/15EngineElec.html

If you let it sit and idle does it smooth out after 30 seconds or so?