1998 Dodge B2500 Van hesitation problem

1998 Dodge B2500 Van, 5.2L, Auto, Air, 150,000 miles
Problem #1 - Each and every time the van is started and accelerated to any low speed, it hesitates or "stumbles" for about 10 seconds. It happens only once and when the engine is hot or cold.

Problem #2 - Accelerating from 50-65mph, on the level w/ no load or AC, takes 15-20 seconds. On a hill or with a load, "pinging" is prevalent. I use 89 octane gas.

Background - The ECM (engine control module, CAT, and fuel pump are new. The ECM failed, pouring gas into the engine, clogging the CAT. I been assured my the mechanic that the timing is on and there are no error codes. Problem #1 precedes the ECM replacement. Problem #2 started with the ECM/CAT replacement.