I agree with Tom -- most likely a simple matter of tightening one side of the chain/cable at the drum and loosening the opposite. Proper chain tension is about 1/8" squeeze between the fingers when finished. Chain tensioning is always done front to back (idler, front fin , rear fin).
Before you do that though, make sure that your pedals are in the same pin holes in the drums, that the drums are perfectly aligned. If both are leaning to one side, work them both to the center; if split more than about an inch, concentrate on the one that is leaning out of position. If evenly split and less than about an inch at the tip, I wouldn't bother -- less than about 1/4" of pedal travel can account for that small amount.
It's rare to have the chain slip a cog. If you suspect this, it should be noticeable. Each sprocket has a master cog in the center and the middle chain link should always be on that cog -- be sure it's pressed firmly in.
Finally, there is a small chance one of your masts is bent. If so, usually minor. Best to leave it if not excessive and make any minor correction with the cable.
While at it, inspect your fin masts (rods) to make sure they are not wobbling -- should be firmly in place. If not, see if your allenhead screw is loose. Bring it in snug to the hidden flat spot on the mast and Locktite in place. If the masts still wobble, they can be epoxied in place (along with the allenhead).