Citroen C3 Picasso 1.6 HDi Diesel Fault code

[LLewop]

Hi.
I have just joined the forum after buying a 59 plate C3 Picasso 4 weeks ago, from a garage in Swindon about 90 miles from where i live in South Somerset, it drove OK no problems with the engine, went through all the traffic problems around Bristol and down the M5 and home was pleased the way it drove.
Went out a few days later up the M5 again to Bridgwater and Burnham on Sea and on the return journey stopped in a pub and parked on a backward slopping parking space, came out the pub about an hour later and started the car, but had the anti pollution fault message come up, along with a low pressure message as well, switched off the car and waited a few minutes tried again and car started with no warnings on the dash, so drove off down the road only for it to stall a couple of miles further on, again waited a few minutes and the car started, so off we went again this time joined the M5 and again it stalled and I was lucky to pull on to the hard shoulder and again wait a few minutes and off we go again, but this time we get all the way home, about 20 miles, without incident.
Took it to my local garage for a diagnostic and was told that the EGR was stuck open he replaced both last week including an air filter and the filter behind the
dashboard. Took it for a drive around the area and it drove faultlessly , so this Sunday took it for a 75 mile round trip everything fine, except when I tried to accelerate in too high a gear with the revs low, although the car didn't stall, the anti pollution fault message has come up and now I have an engine sign in amber on the dash.
So i am wondering if the revs were to low for the high gear would this make the gases in the EGR sluggish, therefore making the sensors think momentarily there was a fault, it carried on driving Ok all the way home. I have read past posts on this recurring fault and wondered if a solution had been found. Sorry for the long winded post but thought it best to give as much detail as possible.

[Huskyxantia]

Ok, different car but this used to happen to the mk2 ford galaxy diesels when driving up or parking on a hill,slope, it was a turbo thing, and when you go back a bit further there's an egr, so it might be worth getting the turbo looked at or tested & the egr cleaned. Hope this might help or shed some light.