I have a 1990 740 GL wagon that has recently developed a starting problem.
When I first start it up in the morning, or after it's been sitting a while,
it turns over basically right away. However, sporadically (and with
increasing frequency) I find that it has become hard to start after I've been
driving it. Sometimes the starter will crank and crank and crank but not turn
over. After a few tries it will start again. I don't seem to have any
problems after the car is started.

I took it to a local shop that checked all the plugs and wires but didn't
find anything. They couldn't test the fuel pump pressure but thought that
the pump might need replacement for $350. I've since heard conflicting
theories about whether it's more likely to be a flaky fuel pump/relay or maybe
an oxygen or driveshaft sensor.

What do all you experts think?

Thanks,
-Bob