vaccume

hey i have a issue when i boot it with my sr. when i come to a stop after my vaccume on my boost gage goes back to about 20. but my rpm goes to 500 and stays there till i come to a complete stop and then it goes back up. anyone know the problem, thanks, also my lights dim a bit at 500 kuz less power.

any help willl be greatly appreciated

chris

sorry edit: my poor spelling .