2001 BMW X5 Engine throttle response issue

I have an after market code reader (Bav. Autosport). The engine on my E53 (2001 X5) will start and run while cold, then starts to decrease idle and quit. After re-start, there is rough throttle response (Will not increase beyond 1200 RPM). The code reader states:
(6d) - Throttle Valve Control Circuit
(E4) - O2 Sensor Limit 1-3
(E3) - O2 Sensor Limit 4-6
I believe the O2 sensor codes are secondary to the main issue. Please tell me what components are involved with the throttle valve control circuit. Or, is it an expensive all-in-one module? I removed the MAF sensor from the circuit to trouble-shoot without significant change to the problem. I do not know if the TPS is part of a modular assembly. Thanx.