Just b/c I'm curious....why are you running this spacer?
Did it do it before the spacer install? Is this doing it when cold? Warm? All the time?
If you want to see if its not the spacer causing this I would:
First make sure there's no air bubbles in the coolant system by burping/bleeding the system. This is often a problem if you've unhooked the coolant lines recently and its an easy fix
If its doing it when it only cold (which by your description, it's not). I would replace the FITV (although its often clean/screwed back in or simply bypassed)
If its doing it when its warm only....I would take IACV off and seeing if its the problem (dirty....broke inside, etc)
Another try would be to check FITV by taking intake pipe off then get the car to running temp (thermo opens). Inside the TB, towards the firewall, there's a access hoel for the FITV....plug it with your finger and see if there's suction. You should have no suction on your finger at proper running temperature. If there is suction...replace or fix FITV (Helm and Honda say to simply replace it but others can contest to fixing it) There about $70 OEM for my B16A (online & delivered)...I just bought one.
If none of this works....Any vacuum lines loose or bad? New TB gasket? Simply.....check for leaks.
Or....take off the spacer and see if that's a fix