Unanswered Hot End Temperature

I am having trouble getting my material to extrude properly. I'm using 1.7mm PLA - measured with a caliper, and entered into Slic3r. I've installed a new EV3 V6 hot end and a Titan extruder, but it seems that no matter how hot I set the temperature, my filament won't go through the nozzle with any consistency. I'll put a series of 10mm marks on it, then hit the button to have the extruder crank out 50mm. The first 10mm come out fine, but then the output slows down and it starts to get chewed up. If I wait a couple seconds and do it again, I see the same thing. It really makes me suspect that the hotend just isn't up to melting the plastic fast enough, but I've had it set at temperatures up to 270, and it's still not working well. I've run a thin guitar string up into the hot end, and I don't think there's a clog. Could my thermistor be wildly misreporting the actual temperature? Is there something anyone can suggest that I try? Thanks...

 

Thanks - I ended up taking the nozzle off to have a good look, and I think I might've melted the PTFE tubing where it comes out of the heat sink into the hot end. It's keeping the filament from making its way through. I'll be back after I replace it. I appreciate the help.