I have a 1999 Chevy Silverado 4x4 with 64k miles, 5.3 L engine and automatic tranny. The truck "stumbles" at different times. The problem generally occurs when I first start the engine and put in gear. Once I begin to give it more throttle, the problem starts to dissipate. During acceleration at low rpm and slight throttle it also stumbles/bucks. The truck seems low on power in overdrive but when it kicks down, it seems to be ok. I have changes plugs and fuel filter. No trouble codes or check engine light. The dealer was unable to find anything wrong...it was working well when I took it in. Any Ideas? I can recreate the problem by holding the brake and throttle, loading the engine. Mostly this is noticed under 1500 rpm.