Versions tested where this bug occurs:
MineFactoryReloaded-[1.7.10]2.8.1-174.jar
MineFactoryReloaded-[1.7.10]2.8.2B1-188.jar

Versions tested where this bug doesn't occur:
MineFactoryReloaded-[1.7.10]2.8.0-104.jar

CoFHCore-[1.7.10]3.1.2-325
Forge: 1.7.10-10.13.4.1558
Java: 1.8.0_66
Singleplayer.
No other mods installed.

Whenever a conveyor belt tries to move an item uphill, it gets stuck.

Revert to an older version of MFR and everything works as expected!

Working as expected (v 2.8.0-104)
https://youtu.be/wp0SOxEWr0M

Not working (v 2.8.1-174)
https://youtu.be/MUx1b7GDWyQ

I've noticed that this happens only when items are being moved uphill North and East. Visually, it appears the items sink into the conveyor belts, compared to when moving South and West uphill the items appear to hover farther above the conveyor belt.

This is a persistent issue, affecting all versions of MFR. 2.8.1 changes the logic in an effort to reduce the visibility of the issue as much as possible, but this is a fundamental issue in the way minecraft handles collisions with blocks. The item isn't moving because it's not touching any of the conveyor belts, it's sitting on the air above the sloped conveyor as far as its code is concerned.