ok so just recently my 87 fc has started to act funny. when i start it up from a cold start it goes to 3k which is normal to speed up the warming process but then it slowly drops down under 1K and then under 500 and then it stalls. zero is telling me its a vac problem but i am not sure. any ideas why it does that?

also it only does it when in neutral and not in gear

 

what modifications have been done?

 

none complete stock '87