JK flip flop integrated circuit (7473) toggle malfunction
If you are really using an old original bipolar TTL 7473 (with no additional letters in the middle), you really should connect the switch between CLK and Ground, and the resistor from CLK to +5V.  

A TTL input sources current, and your 1K pull-down resistor may not be low enough to have the CLK input reliably seen as a Low when the switch is open.