Gas and Oil Home Heating Furnaces - Lennox G12E Problems

I have an old Lennox G12E, with the electronic ignition. The main problem I have is that the furnace will start and run fine, but periodically it will not start again after the temp drops. The problem was easily fixed in the past by changing the blanket filter. Now, after changing the filter it still occurs. I left a little more length on the filter and am wondering if that excess blanket may be bunched up and prevent proper function. Also, it starts up just fine everytime mannually by shutting the thermostat off and restarting it. Don't know if there could be a problem with the temp guage of the thermostat.

Any advise would be appreciated.
Nick

Sounds like the flame sensor needs to be cleaned...

http://www.lennox.com/pdfs/manuals/Lennox%20G12%20Manual.pdf

On page 3, it shows the flame sensor.. Take a green scor pad and clean the sensor..

The heat exchanger on these units are known to fail.. Have you had a clean/tune done on these??

I will give that a try, thanks
The furnace was serviced and cleaned two years ago. I got lazy this year and didn't get it done yet.