Might not be an issue with the vac per se.
The pressure will drop during a flush as instead of applying a vacuum to hold material in the heads its applying pressure to push out old material.
When it does a purge can you see the material droplets coming out of the heads?
If so, and you’re not getting a ‘heads filling error’ then your vac and pumps are probably not the issue.
It could be an issue with the encoder or encoder cable, or communications between the server pc and printer.
Have you tried a pattern or weight test? what were the results?