Mac Mini server - VGA screen issues

i have got new mac mini server & bought mini displayport vga adaptor plugs thunderbolt port. using existing dell monitor has vga cable.

when switching mac on screen show no problems.

if mac goes sleep or if switch off screen takes long time screen respond when return.
moving mouse/keyboard etc know mac responding can't show on screen.
if unplug adaptor thunderbolt port plug in again screen display shows takes @ least 2-3 minutes - longer. quicker turn off every time & reboot screen show instantly.

ideas on how solve this?

 

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