surburban propane furnace problems

[PIExpress]

Hi!
I have a suburban furnace that's acting up. when I turn it on, it comes on, heats (blows hot air) shuts off normally. After the initial running it does not come back on when the temp drops below the setting. It just started doing this. It has worked properly in the past. To get it to come back on, I have to shut it completely off then move the lever back to warmer and it comes back on and runs/warms and shuts off again. Where should I look first to trouble shoot? Thanks for any suggestions.