Old Athern help

Newbie here has a question.

I just got back into trains thanks to my 4 year old son.
I have a Athern Alco PA made around '77 my dad bought me new.
I just had it serviced (new motor mounts, drive shaft, cleaned wheels)
and although it runs good, it's seriously lacking some balls, it only pulls about 10 cars and seems to need to warm up a while before it will pull.
I admit the curves are a little tight, but it seems this being a larger engine it should pull more then it does. It also seems a noticeably slower. Any suggestions ?

Thanks