Lately i been experience some idling issues when starting up my 99 Nissan Sentra SE-L. When I usually start up the car the idle drops down from 400-600 rshared_pm. It stays like that for maybe 45 seconds then goes back to 1200 rshared_pm.Today I was driving to work and I was getting on my car and I notice a cloud of black smoke came out of the exhaust. Can this be a tunning issue? Or something else?

Things I have one to the car:

*GTI-R motor
*Weisco forged pistons
*Eagle rods
*ACL bearings
*GT30rs turbo
*550 injectors
*Z32 maf
*Ecu tune from JWT
*Neo safc
*NGK iridium spark plugs
*NGK spark plug wires