98 cobra idle

hey guys. minor thing that bugs me on my 98 cobra. occasional low idle.

i can start the car, leave it idling for around 30 seconds and then the revs drop to around 400rpm then idle low for around 5 seconds then idle properly, even within the choke period. if my lights are on the idle speed will drop but the dashboard lights seem to dim as well, as if its not earthed properly. voltmeter drops ever so slightly when it does it, but i was told it was because of the low idle and is normal.

its only happened since i changed my plugs over. it kept dying out whenever i dipped the clutch, but i disconnected the battery overnight because of suspicious activity and it didnt stall anymore, but did this. i havent done anything else to it for it to do this. any suggestions?
 
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