Tom;

Wierd, seemingly unexplainable electrical problems (in the end, electricity is TOTALLY predictable, and explainable!), can most often be explained by poor ground connection(s). That is when everthing is as it should be, the ground connection is SUPPOSED to be the return current path to the battery and charging system. When one of these ground connections becomes compromised, electrical current might flow through some totally unexpected path, connecting two items which would normally have ZERO to do with each other (other than being in the same car!). This is what then causes the totally whacko symptoms.

In your case, light current may have been flowing through clutch cable (if your car is so equipped), associated with a poor chassis to block connection...step on clutch, lights might do strange things....this MIGHT even happen with a hydraulic clutch!!! The sluggishness on start-up, may be traceable to a poor spark due to low system voltage (or forgetting to release the hand-brake)!

Its tricky to remotely troubleshoot electrical problems...fortunately though, the electrical system of a vintage Volvo is fairly simple. Some general suggestions:

First, assure all the connections having to do with the battery and charging system are clean, free of corrosion and snug. This includes the braid, connection from chassis to engine, connections at fuse block, and at ignition switch. Next, monitor the voltage of the electrical system (at ignition switch for instance), a temporarilly connected meter taped to the dash will do it. Report back, if problem persists, noting ALL symtoms.

Good Hunting!