21-4BTT with 2084 and 41021 header and Nova #6 plug.

My symptom is when I come into the pits once the engine is fully heated (I use a competition heater before it's started also) or after the run the engine will drop to a nice idle instantly and then after 3-4 sec revs back up. After 8-10 sec of being reved up at a the higher idle it idles back down.

I would assume this isn't normal and it's not an issue I'm familiar with. Any suggestions? Thanks.