Flat spot after long descent

[TONYT25T25]

DG1.9 Water cooled. Just returned from Lyme Regis and after a long descent down the road and then onto a relatively straight flat road, the engine struggled as if on an incline changing gear did not improve the situ. Did the same journey last year and the same symptoms were experienced in the same area of road. Last year I pulled over as I thought the engine would cut out. Tank was half full of fuel on both occasions. Any advise welcomed apart from don't visit Lyme Regis. Have read the wiki tips but wondering if other issues may need looking at. Tks Tony

[T25Convert]

How long did the lack of power last for?

Was it coughing and spluttering or running fine but with reduced power?

[TONYT25T25]

I don't recall any coughing or spluttering just no power increase when depressing accelerator pedal. I have no rev counter, but there was also no increase in engine noise/acceleration. This lasted for about a few minutes, then back to normal operation.

[patrickpawsey]

Blockage in the tank caused by something sliding forwards towards the outlet and restricting it on an incline?

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[TONYT25T25]

Odd thing is that it only happens on this stretch of road and not when going in the opposite direction ie into the town where there is another steep road leading down, although this is interrupted by give way to oncoming vehicle gaps. I am wondering if not using the accelerator pedal whilst going down the hill is a factor as I was going fast enough without the need to use it. Another way to describe it is using the wrong gear going up a slope ie 4th when you should be in 1st or 2nd.