nozzle cloging

hy,
i have just finished building prusa i3 with ramps1.4
so i am new to this forum and new in 3d printing
i have some odd problem with extruder or hotend my extruder grinds fillament in half printing and stops extruding since i have wade reloaded i have tried changing hobbed bolt and i have made 12 variants of hobbed bolt but this is hapening again and again and then in one moment i dont know what i have done it starts printing fine and i started calibrating the printer... and after 20meters of filament printer again have stoped working so i have clean nozzle by burning it.
i have clean nozzle and change nozzle with other but no effect.
then i tried to extrude by hand and on start it is extruding bůt if i slow down or stop extruding for moment and then try the nozzle is cloged. after atomic pull i can easily extrude 10cm of filament but if i stop pushing for moment to exchange hand it clogs and after atomic pull i can again extrude but just 10cm of pla fillament.
i tried so far
i tried changing temperatures in range from 185 to 260 no effect
i tried chanfing nozlles with new one (0,3mm)
i tried changing hobbed bolt
i tried push plastic on heat block to see if temp is ok and it melts.. (so i guess that tem is ok)
sorry for my english
can someone suggest me some kind of solution?
thank you

What make of hot-end?
Do you have a fan running on the heat-break of the hot-end all the time?

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My updated Instructable on our Prusa i3 Build
[www.instructables.com]

I have j-head v6 with 0.3mm nozzle with fan
i dont tink that clog is on the barrel but on nozzle...
when i push filamment with hand it goes easy but when i stop pushing for second or two and then try to push i cant extrude anything enymore and then i perform atomic pull and i can again extrude by hant until i stop pushing and then is clogged again until atomic pull

it may be your filament. Sometimes cheap no-name brand filament can have air-bubbles or be inconsistent in width and roundness. If you have access to another printer try your filament in that printer, or if you can borrow filament from someone else who has already proven that it works in another machine.

I recommend hatchbox filament. it runs about $25 a role, and is of good quality. There is a printer farm at my university, this is all that they run through the 8 makerbots and i have not seen one jam.

Thank you for fast ansvers
i still dont know what was problem
after all things that i have tried i decided to put a drop of sunflower oil in nozzle and then it started to print but 2-ë prints after doeant stick to bed after 2-3m of fillament now it sticks and works ok
thank you

Make sure the extruder stepper motor is calibrated properly, what I did was remove just the nozzle then get the filament flush with bottom opening in heater block which I then moved 200mm up from bed exactly..Then you manually feed the extruder till the filament touches the bed and it should show 200 mm not 190 or 205 but 200 exactly otherwise you need to change the steps/mm for the extruder stepper motor mine was done through eprom not sure on yours. Repetier has a tutorial on calculating those steps and procedures. That solved my nozzle clogs. And make sure layer height setting is less than nozzle diameter size

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rwt