It may not be your glowplugs but your timing as your crankshaft pulley may be worn. It is critical that you remove it to check the keyway on the pulley for wear. Mine did this, and before I checked it, destroyed the motor. It was only noticeable when cold. Should have listened to my wife when she said it was running rought. Thought it was glow plugs also, so left it for a few weeks.
If there is any wear, you can not just replace with a new pulley as the crank will be worn also.