I've had issues with both the tps and iac, but they are easy to replace. Tps caused a strange idle. Iac caused hard starts. Both issues were easily remedied with a trip to Napa and using a 99 Yukon as the automotive equivalent.
My worst problem wasn't even engine related. It was caused by the exhaust silencer choking the engine st about 3200rpm.
2001 X-Star 5.7l LTR