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I put together a design for Express Underground Belt production and let it run for a while. I noticed that production was stable, but then there would occasionally be a halt in production, then it would quickly recover and produce a lot, then return to normal. I took a look at the Express Underground Belt assembling machine and noticed that the stack inserters feeding it were only feeding it gears during that period of halted production, up to 1000 being in the buffer, despite there being available Fast Underground Belts to use for crafting. Pictures attached.

Only mod used was Creative mode so I can easily feed iron plates into the setup. That energy source and one chest on the left are the only modded elements in the save.

This is working as intended, the input slots are expended significantly as the assembler is running very fast and you do not provide enough gears so the inserters are busy inserting in small stacks each haul, so other items are not inserted and production halts.
To improve the behaviour have each chest only provide one sort of item, do not let the chest run out so inserters are most efficient and increase the number of chest. NaB.