Turn signal goes out at "high" load

Hello folks

I have bought my first US car ('71 Dart, stock 318) a few months ago and currently just enjoy driving it around. As temperatures started to drop I found out that

• heater works (... there is a slight temperature increase after 15 minutes of driving... :) )
• heater fans work
• all lights work (except for the dash, but I messed them up after messing around with an aftermarket tach)

All seems well, BUT, when its all on, as they usually are on a cold, dark morning like today and I roll in stop and go traffic, the turn signal will only blink very slowly. It doesent go on and off at the usual ~1 second interval but 2-4 seconds at an irregular pace. Twice it went out entirely but came on as soon as I started accelerating a little bit.

Before I start looking for problems where there are none: is the above behavior normal or is there something wrong with some part of the car?