Thanks so much ... Cleaning the pick ups was exactly what my slow loco needed.  Thanks for the advice!

Hi everyone!  New to the forum here .. And new (or, maybe I should say back from a 35 year hiatus) to O-gauge railroading!  Glad to be here ... Go easy on the new guy!   
 
In early December 2012 I bought my 4 year old his first real Lionel set - the Santa Fe freight set (6-30173), with the hopes of introducing him to this fantastic hobby, the same one I stepped away from 35 years ago as a little guy myself.  Naturally, Owen was over the moon to receive his first real Lionel, and has been enjoying it A LOT over the last 10 weeks.  However, over the weekend the locomotive started acting rather funny, running intermittently and really struggling to get through the medium sized layout.  It's been running fine, and my locomotive (from the 70's!) also runs great on the tracks, so I know its not matter of having a short, a poor track connection, or dirty rails (and yes, I did clean them).  Any suggestions of what could be making my Santa Fe loco run so erratically so suddenly, after only 10 weeks?
 
thanks, Jim Richardson (on the Providence & Worcester line!)