Check the connections to the indicator bulbs, especially the rear ones. They can corrode, but even a dirty connection can cause a problem.
The flasher unit contains capacitors and transistors. A transistor is a voltage controlled switch, and the capacitors fight to prevent changes in voltage (which is why they are used for smoothing in some applications). When used together, the capacitor controls the voltage in part of a circuit, and when the voltage exceeds a certain level it allows current to flow through the transistor. By cycling the charging and discharging of the capacitor, it allows the transistor circuit to open and close. If the speed of this cycling changes, it means something in the circuit has changed, allowing the time for the capacitor charge/discharge cycle to decrease.
It could be the flasher unit that is causing the problem.
Maybe the alternator and battery voltage is too high (less likely, but easy to check), or the resistance of the circuit has changed. The resistance can increase because of a failed bulb (which you've already checked), or dirty or corroded contacts somewhere in the circuit, including relays and so on.
Something else to check is that all bulbs have the correct power rating. The E30 has (I think) standard 21W bulbs.