As noted above it's time to disassemble the motors and trucks and clean everything. Clean the brushes and springs and if any look worn replace them. Clean the commutator as well. Grease and oil the motor with your preferred products. Same goes for the trucks. Remove all the grease and oil, make sure to reassemble everything correctly, and re-grease and oil everything. Once completed the locomotive should run like new and have very good performance throughout the throttle range. It should be flying around 12 volts or so.
I've got a 1951 vintage set of 2343s. They're the only Postwar locomotives I own. The A units have had the frame repainted and Lionel Conventional Classics shells replaced the worn Postwar originals. The powered A unit has an ERR AC Commander and I added a Railsounds B unit and another modern (90s) era B unit to the set. They run beautifully for what they are.
