Blobs forming on nozzle destroying print

I'm stumped!! I've printed about 5 of the same thing sliced with different settings in slic3r but couldn't get a usable print.

the first 2 layers always prints beautifully, but after the third the problems start appearing and just gets compounded as it goes. it looks like blobs starts forming at the nozzle and as it prints, the blobs get caught onto the print and start building up bigger and bigger. see attached image.

Printing on Mendel w/ Gen6/Sprinter, 3 mm PLA filament, mendel-parts v6 style hot end w/ 0.5mm nozzle

Sliced with slic3r 0.7.1, settings as follows

Layer height - 0.4 mm

Fill density - 0.2
Fill angle - 45
Fill pattern - line
Solid fill pattern - rectilinear

Nozzle diameter - 0.5 mm

Filament diameter - 3.0 mm
Extrusion multiplier - 1
Temperature - 160 C
First layer temperature - 170 C

Print speed
Perimeter - 24 mm/s
Small perimeters - 24 mm/s
Infill - 48 mm/s
Solid infill - 48 mm/s
Bridges - 60 mm/s

Travel speed 64 mm/s
Bottom layer speed ratio - 0.3

for extrusion width (ratio over layer height) I've tried 0 (auto) and a calculated value based on measurements from a piece of extrusion, result being 1.157894737... but there was no noticeable improvement.

So this happens after your temperature starts dropping? Have you measured your filament? I have yet to get a spool which measures exactly 3mm, usually they are closer to 2.78 to 2.85, and I've gotten similar erratic results when they are not consistent along the length. The hobbed bolt ends up pushing different lengths out the nozzle when the system thinks it is pushing the same amount. Since you say your first couple layers look acceptable, try raising the print temperature to the same as your first layer.

the filament was measured with a digital caliper to be between 3.0 mm to 3.1 mm. got them recently from mixshop.

before this latest failed print the previous prints were printed all the same temperature throughout the layers. so i don't think that's going to cure my problem.. i dropped the temperature cuz' the object remained soft for a while and didn't provide a good base for the next layer, so i lowered the temperature.