ok, sounds like you should treat your engine to a valve adjustment, but i dont think that is your problem, id first check the glow plugs. the best way to do this is to trace the g.p. harness from the engine, back to the g.p. relay on the left front wheel well. all the wires that hook to the g.p.'s run into one harness at the front of the engine, trace it on back to the relay. next remove the cove on the relay, and unhook this harness. the g.p. all come to a common contact internally in the relay. you must unhook this or you wont get accurate test results.
next take your ohm meter and hook one lead to ground on the engine and take the other lead and put it on each g.p. terminal. the reading should be anywhere from 0.1 ohms to 0.9 ohms, any higher resistance than that means the glow plug is bad and should be replaced.
now, i know there are alot of cheaper glow plugs out there to be bought, (autolite, champion) etc. but i caution you against these. if you will look at the oem "bosch" plug you will see it operates at 11.5 volts, these others operate at 12volts, the lower voltage makes them glow up faster and also get hotter. result is a faster more dependable start. i have also noticed that the non bosch glow plugs are not near as durable, or long lasting. good luck my friend!!!!