Mercedes sprinter loss of power and black smoke

[Daron 0]

Hey guys not sure if I can post my car problems here but I’ll give it a try anyway, my Mercedes sprinter 2008 315 CDI has been having problems lately with losing power and a lot of black smoke coming from the exhaust... my local mechanic did a forced regen on the dpf plus a second hand transmission has also been put in. The air filter is fine also.The mechanic checked with a code reader and he said it was a dpf problem and was blocked 60% but after the forced regen it went down below 5% which was good.however today when I drove It was driving  fine for about 20 minutes and then the engine light came on and starts flashing and as soon as this happens the car loses all power. I was going up a hill on the highway doing 40km/hr in a 110km/hr zone With my foot fully on the pedal and no power at all!
my mechanic said it isn’t normal for it to be losing so much power and I’m bringing the van back to him soon but I’m curious myself with that’s happening. He said the engine light flashing is because it’s doing a clean through the dpf but He said that it should lose that much power. If you guys could help me at all with what you think is wrong would be much appreciated! Cheers!