Silvertop Idel Problem

[XSVTOY]

Well I got a 92 FXGT it is basicly standed

The engine Mods are TRD leads and HKS Foam Filter

I dont know if it has any internal mods but it wont idel properly when its warm it kinda sounds like its got a rotor under the hood the rev counter jus bounces between 500rpm and 1000rpm, Anyone hav any idears why it does this? it idels ok when cold but when it warms up it goes funny

Also what oil should I be useing in it as I jus put Castrol Magnatech in it!