KM C500 image quality issue on gloss

[starship fighter]

Hi there,
We run a pair of KM C500's which, 99% of the time are rock solid. the bulk of the work that we do on them is posters and such, for which the customers demand a decent presentation finish which we get by using Howard Smith High Tech gloss (as recommended by KM).
However from time to time we get an odd mottling effect on certain jobs. it seems to creep in at about 30 prints or so, and it only ever happens when using the gloss paper. We've tried about 4 different brands of gloss and the problem seems to occur on all types, but the headscratcher is that it doesn't occur on some jobs at all, yet on others it's a problem throughout the run. What is even more bizarre is that if prints are sent down in short runs of say 20 or so it doesn't happen at all, only on longer runs, which kind of suggests that it is a temperature issue. It doesn't seem to make a difference though if we alter the speed that the sheet passes through the fuser.
Has anyone else come across this problem? We've had several engineers look at it but they have all drawn a blank - advice given has been at best 'well don't use gloss then...' which doesn't help when that is what the customer wants!
I would be most grateful for any advice you could give!

All the best!

[Livebychance]

Let’s say job A causes a problem and job B does not.

Can you run job A at anytime (a week later) and have a problem?

Does job A have allot of saturation?

Can job A and B swap problems?

Put a line monitor on machine and when problem presents itself view the details.

[lucky13]

what is the g/m2 ? (waight / thickness of stock) which paper drawer is job sent from ? what is the paper tray setting set at ? and by mottling is it unfused toner in high density areas or tiny little cut marks on the edges ,maybe you can upload an atachment PDF or jpeg so we can see it .

[random]

Change the grain of the stock.