Engine Management Light ?

Hi, my freelander Td4 es 76000 miles has a slight problem, when driving at no set speed the engine man., light comes on ( the one with an ! mark in ) with a slight drop in speed followed by a small smoke plume from the exhaust, the light then goes off and after 2-5 miles it does the same again. I have change the cam pressure filter and the main air filter and added redex diesel cleaner but no joy. Its been in to a landy dealer for diagnostic but shows no fault codes. When free revving at standstill this doesnt occur? Is this injectors or sumthing else? Amo

Sorted this problem, when fitting the EGR bypass valve I decided to remove the inlet manifold to clean out all the gunge, I found the nuts were loose to the front of the manifold housing, (square inlets) and could be removed by hand. After mucho cleaning with Halfords foam degreaser( lemon flavour) I ordered a full set of 8 gaskets/ seals 4 round 4 square from island 4x4. Big difference now, quicker off the mark with no sudden drop in power and black smokey exhaust and no engine manage light. Next is the oil and filter job for weekend. No rest for the landy owners.