Hotend Temp Drops During First Layer

I am trying Slic3r for the first time and a test part (a L-shaped bracket) printed out very nicely.

Now I am on to something a bit more complicated. Both times that I have tried to print the part the hotend temperature has started dropping half way through printing the first layer. Both times it happened at the same point. I don't understand why.

I am using Marlin 1.0.1 RRP, pronterface and Slic3r 0.8.4.

In Slic3r the temperature and first layer temperature are both set to 185C.

In pronterface I pre-heat the hotend to 185C before printing.

I am printing from a SD card.

I checked the gcode file (2MB in size) and the only M104 commands are at the beginning and end. After printing starts there are no "M" commands at all until the end. The only "G" commands I see are G1 and G92 E0. I tried manually entering G92 E0 and it doesn't shut down the hotend.

Any ideas on how I can debug this or where to look for clues next?

Thanks, Andy

Can you rule out a bad electrical connection of your hotend heater?

Yes because this morning I've printed three calibration blocks plus my wife's name on a plinth without problems.

Maybe it is a bug in Marlin?

Andy

Marlin can shutdown your print if thermistor reading is zero but it does that and halts with an error message. I guess this is not your case either.

Could you try removing Slic3r temperature control altogether (emptying the temperature boxes when slicing)?

I'll give that a try however I've now printed several different parts, some of them multiple times, all using gcode to control the temperature at the start and all print without problems, so I think it is related to that one design for some reason.

Andy

I've seen some low temperatures on my printer using Marlin that were too low to be true (as it was still extruding ABS below 200C). It turned out my thermistor was loose but the temperature averaging code (I guess there is some but I have not looked at the code) was disguising the problem for a few seconds.

It eventually ended up halting the system due to bad temperature sensor readings, but for a while it looked as if temperature was lower that reality.