TeamSwift

My car recently started a high idle every time I come to a stop. 91 vert auto trans 300k miles. It reacts as though I just started the engine and its cold even though it's up to full operating temperature. I just pop it into neutral and it slows down to a normal idle in about 10 seconds or so.

My mom is going to borrow my car for a couple months and she's not going to pop it into neutral like I do and I'm worried about it burning out my antique transmission. So I'm looking for suggestions of what you guys think maybe causing the high idle. 2000rpm

Also, after the engine slows down and I shut off my dash fan it'll rev back up to 2000rpm.

Bone stock unaltered engine except for ignition coil.