G270 CDI …. Problems in stationary gas

Hello all, my G270 CDI, 42000 km Bj 2004 has been having problems with the stand gas since the latest. To the info… a year ago a new Uylinder head was mounted and recently a new gearbox with converter. When I drive towards a crossing and brake to a standstill it suddenly ran out of me twice. The stand gas is often no longer like a clockwork, but is always a little sagging. It is not always, however, increasing. Has someone ever had comparable problems? It starts great and when driving you don’t notice anything. Only in the stand. I would be very happy if someone could help me. Best regards Hans