You might be using lower wattage bulbs, or even LED cluster type bulbs, so if all your new bulbs are ok but flashing fast, then you will need to put an extra load on the indicator circuit which you can do in several ways, you can buy a big watty resistor something like 12 ohms 12 watt ( actual ohms depends on what load is there on the circuit now so this is just a basic guess and will have to be by ttrial & error ) but as I said add a 12 ohm resistor this should add approx 1 amp load to the indicator circuit, or you could add a 10 Ohms 15Watt resitor for 1.2 amp load, these resistors can be expensive, and alternatively you could use the standard indicator bulb wired across the indicator circuit inside thye engine acting as a load and neately and safely tucked away ensuring that it doesn't short against anything and secured properly in a proper bulb holder, leaving the bulb part uncovered i.e do not wrap any tape on it to block light as this may get very hot and a possible fire risk.
I think Dom suggested something similar like adding a resitor across recently to another member who had the same problem, I am not sure what resitor value he came up with.