AEG Washer/dryer Dryer stopping after 19/20 mins

AEG L-16830 91460212000
Hi,
I went a couple of weeks ago to the same machine and the pipes were frozen, no problems. The guys tells me about a problem he has with his dryer . He can put it on the synthetics dry and it works fine on both timed dry and sensor dry, but on the cotton dry it works for about 19/20 mins and then goes to 0 and finishes. I tried it and of coarse it worked fine for me but i could only try it on the 30 mins prog and the last 10 would be a cool down.

The guys called again tonight to see if I can have another look at the dryer for him.The machine is not flashing a fault code either to suggest it has overheated etc.

Has anybody else had this problem before or can you point me in the right direction.
I have before now had problems with the water valve stopping working after a while and also the condensor getting blocked up, but why would it work on low heat drying and not on high heat and both of these have flashed up faults in the past.

Anyway if anybody can point me in the right direction it would really be appreciated.

Cheers

Nick