One burned out lamp (on a 2 lamp circuit) could cause a slow blink with long time on while the bi-metallic strip slowly heats up enough to break the circuit and then the same as normal quick off time.

Or ....

The one remaing good bulb may not use enough current to make the bi-metallic strip bend enough to ever break the circuit. This results it the good lamp coming on and staying on.

I've even seen a combination of both symptoms. When approaching a red light to make a left turn the turn signal is turned on. Engine speed is around 1000 rpm and the voltage from the alternator is around 14. 5 volts. At this voltage there is enough current to cause the bi-metallic strip to do it's job slowly. But as you pull up to a stop the engine speed reduces to a 500 rpm idle and the alternator output drops to 12.6 volts. Now the current flow is reduced below the threshold necessary for the bi-metallic strip to bend enough to open, leaving the one good light on all the time .

I'll bet your headache is worse now Kevin.

'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.