You've likely got two problems going here.

First off, when one headlight is dim like you mention, it's a sign of a poor connection. In many cases it means there's a poor chassis ground connection, as others have surmised, so that's something you should definitely check, but in your 740 I'm willing to bet it's more likely the headlight connector itself. The 700/900 headlight connectors are well known for deforming under heat which allows a weaker contact and makes for a dimmer bulb. Once it starts to happen it generally tends to just keep getting worse until you eventually have to cut the connector off and splice in a good used one. If it's just begining to happen then you may have success trying to improve the connection at the back of the bulb. Use contact cleaner in the socket and carefully tweak the contacts a bit so they're pinching nice and squarely together. Similarly clean the bulb terminals (avoid handling the glass on halogen bulbs leaving oily finger prints). Apply a little dieletric grease or a Deoxit product to help prevent future corrosion. Reseat the blub as carefully as you can on the connector and mount it back in the headlight assembly then peel the boot back and push the wires in from the back of the socket to make for full contact. If the socket isn't in too bad shape and you do a good job it should last if you can just remember not to accidentally pull on the wire. The problem with high wattage headlight bulb connections is that once there is poor contact, either from oxidation or the connector not being fully seated, the metal contacts will heat up and start to soften the rubber socket such that the female contact is no firmly held in place and additionally the copper fingers themselves get weaker. Use of higher wattage aftermarket bulbs in an attempt to gain better road lighting only make matters worse for those 700/900 connectors.

The second problem you've likely got is an old weak battery that can no longer hold a good charge, possibly not even deliver full voltage. An auto service center can do a load test to check the condition of your battery, but if it's anything much beyond 5 years old then the battery should be considered on borrowed time. For newer cars with lots of electricals, anything over 3-4 years is now considered borrowed time, especially for a cheaper battery.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now