thermocouple are sometimes tricky to read. I would suggest you to read Microchip AN684 and AN844 appnotes. You should find some interesting information. Also the following should be familiar...
http://www.delabs-circuits.com/cirdi.../doc00024.html

If it was me, i would use a dedicated Thermocouple A/D convertor instead... but it's me..

If you don't have any scope to see what's happen on the supply line and on the LM324 pin... it's like fishing in the dark...