2003 GMC Savana Engine Stalls

I have a 2003 GMC Savana 1500 5.3 V8

The engine stalls whenever I come to a stop or a turn; or slow to about 25 mph. Hot weather makes it worse. I researched some of your responses here. I do not have an idle speed control motor on this model as told by an autorepair store. A mechanic told me it might be the fuel pressure? Another told me it might be an exhaust/hose leak?

I replaced the battery; air filter; changed the oil and all the fluid levels are full.

Sometimes, the warning lights (battery, abs, oil, check engine, etc) pop on then go off again.

Everytime it stalls, my battery gauge dips too.

If I keep my left foot on the brake & right foot on the gas, this prevents it from stalling. While sitting in park, it will idle just fine. While sitting in Drive with my foot on the brake, it stalls and the battery gauge dips.