2001 Cavalier

2001 Cavalier 119,00 miles 2.2l engine. Engine hiccups and shudders at any speed, worse around 40 or 45 mph or above when the engine is warm. Have replaced plugs, plug wires, taken to several garages including Chevy dealer. Not throwing any codes when hooked up. Used fuel injector cleaner and injectors have been checked and ok. Fuses are okay. Serpentine belt is fine. Dealer replaced a connector and it is no better. Does not stall out but scary to ride in. Any ideas? Dealer noticed that sometimes some of the cylinders were not firing but ???? how to fix.

I'd start by checking the ignition module and coil packs... The dealer didn't check those?

I know that the dealer checked the ignition module but not sure about the
coil packs. I will check. Thanks.