I would leave or not leave...not halfway. The supplier is reliable, or it isn't...
If leaving, I would vet the new supplier over several shipments (lots) prior to contacting the old supplier to tell them why you left.
Risk management: If you leave to a new supplier, tell the old supplier that they have quality issues and that's why you're leaving, then find out the new supplier is less reliable...then go back to the old supplier...I would expect equal or worse quality, and a higher price...
Make sure you're in a boat that doesn't leak before you tell the old boat's captain that his boat leaks too much.
There is no requirement to tell your old supplier why you left, but if you ALREADY have a good relationship with someone there (production manager, QC, Sales)...letting that
person know might give them leverage to start improving things. I would not let the
company know...just the
person(s).
Companies don't change things, people do.
I changed heater suppliers twice in 3 months.
The old one was poor quality and slow.
The new one was fast and accurate....for the first shipment.
The second shipment was 2x slower than the first supplier, and they charged me a second time to make up for unforseen expenses before they would ship.
I had no choice...I needed the parts.
I had to spend a lot of time and angst finding a third supplier...that's why I caution you to get several shipments from the new supplier before committing.
(FWIW, only the second {worst} supplier was ISO9000 certified)