So a 75W lengthened it to...4.5 s? The difference is kind of small after all.
@CDU certainly stock up on one or two replacement bulbs. You never know when you need them. Two is better than one.
In case it turns out to not be the bulb: the Durst power supplies are getting long in the tooth and fail frequently. If you have a digital multimeter at hand, please measure the voltage on the bulb socket to verify it's correct. Also check corrosion or damage to the socket as
@ic-racer mentions. If you don't measure power on the socket, measure on the connector at the back of the power supply.