Town bike intermittently harder to pedal
Are you sure you don't have a kick shift/kick back hub in the rear? I had this exact problem after I bought a new, used rear wheel for my town bike. 

Seemingly at random it would be much harder to pedal, sometimes forcing me to hop off and walk up hills. At other times I'd be able to ride up the same hill! I thought I was going mad, and I too thought something was broken.
After a while I noticed a pattern, that it was changing in "difficulty" whenever i braked, ie. backpedaled. Now I love it, easy, hands-free shifting!

Sheldon Brown glossary page: http://www.sheldonbrown.com/gloss_i-k.html#kickback