better motor, flywheel,

better motor, flywheel, alighnment of drive rods, smooth running mechanism, more

Search for articles how to make a steamer run well.  It all starts by taking it entirely apart, remove the motor, too.

ensure the wheels roll freely. 

re-attach the drive rods and ensure it still rolls smoothly.  You may need to quarter the drivers

Check the motor is lubed and runs well.

clean the wheels and everything to ensure good electrical contact.

Then post again and see if it helped.

Loco

Scale?

Brand, model of loco.?

DC or DCC?

Severe cogging could be the motor or mechanism.

A photo of the loco with the shell removed might help.

Rich