THC07

hello, i have a idle problem for awhile, check all the possible problems, somehow i fixed it, but my issues are just different now.
she will idle just fine, when i come up to a light and put it in neutral the rpm needle drops all the way down. sometimes she stalls and sometimes she picks back up to idle. no vac leaks that a can of carb cleaner can find.
tried different ecus. no codes. any advice? TIA