I have an 86 GXL with an S5 block, ECU, and S4 front cover and intake manifolds. Problem is the car starts up and it wants to die, it feels like a large vacuum leak but I have yet to find any. I've sprayed the entire intake area with carb cleaner and no luck. Fuel pressures good, CAS is all lined up and in good working order, TPS has been adjusted, BAC has been swapped, idle screws been messed with, still no idle and chugging/hesitation/sputtering when accelerating even when its warmed up. I'm running 5W30 for initial startup so the oil pressure reads over 70 at or over 3000rpms, doesn't smoke whatsoever and doesn't seem to overheat (according to stock temp gauge). What has me stumped is this loud whooshing sound coming from under the TB when I let off the throttle, any idea what this could be? I've tried everything and I'd like to have this car running soon, so any help is appreciated.

Edit: Also adding that I have a buzzer code for the O2 sensor...