Since these are connections to other switches you don't actually want to have portfast enabled. Portfast on trunks is meant to be used with end devices like IP phones that don't participate in STP (e.g. BPDUs) so that the port transitions to forwarding as quickly as possible, trunks to other switches on the other hand should go through the STP process to ensure there isn't a loop in the network.
So if you are seeing "flushing" a lot, that indicates we are receiving a lot of TCNs or seeing trunk links go up/down. You may want to check the interface stats to see how frequently they have flapped or check the other switches to see whether they have portfast enabled or disabled, if disabled they may be generating the TCNs which we are responding to.
Best regards,
Madani