Trouble with heating printer head

My prints now seem to work great except sometimes during the print the temperature of the printerhead just seems to drop. Sometimes if I'm watching I can often manually reset the heat on and off to get it to respond but then other times it just drops...regardless of my controls. Any ideas on this one ?

Faulty connection, bad thermistor, bad hotend, next to impossible to do more than guesswork without the details...

I hate to admit this but after countless attempts and rechecking everything I realized that the print head always seemed to cool at the same point in the print. I searched the code and found that at a certain point, the print turned on the cooling fan and alas the temperature was reduced to the point where filament wouldn't flow...I add this comment to aid anyone else that may have a similar problem.

thanks again for the quik assist downunder35m!!

I had a similiar problem. The nozzle went to 170° from 195° and the filament was stopped.
I've fixed it adjusting the PID.
Use the M303 command on pronterface (or repetier), it will give you 3 parameters that you have to put in your configuration file.
You can use M303 S200 (if 200° is your print temperature) to have the best result.

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Stampante: RepRap prusa i3 con estrusore bowden
Software: MarlinKimbra

I had this problem until I rebuilt my MK8 by removing the yellow Kapton tape (threw it away forever) and used red RTV high temp silicone to hold the thermistor into the block then I used a larger (set/grub screw) to hold the heater cartridge which I coated with heat sink grease and put in place. And finally I changed the heat sink fins on the extruder fan from vertical orientation to horizontal as per a post I seen. Now my temperature stays pretty constant and drops a few degrees when the cooling fan comes on which can be set for speed. Heater is 12v 40 W when it fails I will go to a 120v 100 W cartridge from Chromalux (requires a relay).