Wade extruder jamming - filament feed rate or hot-end temp to blame?

I'm a newbie and have just finished construction of a Prusa. All the electronics, end-stops, steppers etc work fine. The various hot-ends I tried were a disaster from start to finish - until I installed a J-Head Mk IV-B from hotends.com (Brian made my 1.75mm filament version and rushed it too me - Thanks Brian!) This works great - in that I can manually extrude filament without a problem.

Although I know have a working J-Head hot-end (I can click "Extrude" all day long and it pumps out molten filament), when I try to print something - a bottle-top opener in my case - the extruder ALWAYS jams after a short time. The furthest I've ever got was to part-way through the second layer and the first layer was pretty shabby.

So my question is this: is the feed rate of the filament too fast (if so, where do I change that rate), or is the hot-end not hot enough? (if the latter, how does everyone else measure the temperature?)

I'm using RAMPS 1.4, Sprinter, PronterPrint.

When the printer jams up, in Pronterface hit the pause button, then in the send box (bottom right) type M105 then hit the send button. This reports back the temperature of the hot end and heated bed. If the temperature has dropped off, this may be the hot end over temperature protection switching off the hot end. Switch on smoothing in the firmware.

I dont use 1.75mm filament myself , but I assume you should be able to push ith through the extruder by hand quite easily. Slacken off the pinch wheel and see how much resitance there is.

I'm having to use a fair amount of pressure to get the filament through. Difficult to gauge how much pressure "quite easily" is!

If I hit extrude it comes out generally in I tight spiral below the print head. Could it be my hobbed bolt is designed for 3mm filament?

How much pressure should be applied by the pinch roller?

If you pause the print and wait 60 seconds then resume the print does it extrude properly again? (you may have to give the filament a nudge to get past the bald spot)

If so you are running too fast and the heater can't keep up.