I own a 1991 740 8 valve with about 190000 miles.
This car has always started and been reliable but on Wednesday it would not start right up as it always has. After a few tries, it did catch and started but I've had the same problem on and off ever since. It turns over like a champ and at first I though a new timing belt might be in order although I was up to date at 150,000.
After it has proven to start, from time to time, and runs fine after it fires I am stumped as to what should be done.
My thoughts are the ignition or fuel pump, but I have no idea of how to determine what direction I should go.
Plugs and wires are new.
Fuel is a bit low (onr quarter full) but has not been a problem in the past.
Can anyone help me diagnos?

Check your FPR, and fuel pump fuse.

I would first see if you get spark on those times it doesn't start. If you have spark, then suspect fuel. But that doesn't mean the fuel pump is bad. FAQs has description on how to jumper around the fuel pump relay and see if the pumps run then.
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Whenever you start the car, turn the key to the ignition position first and make sure you hear the pump run for a couple seconds before cranking the starter.

If you don't hear the pump "priming" for a couple seconds, the problem could either be the pump, the relay, or (less likely) the fuse.

If you're getting spark and you hear the pump running, yet it still won't start, I would turn my attention to the radio suppression relay.

Fuel pump can fail intermittently. At 150 k miles, a fuel pump change as a maintenance thing might not be bad and it might cure your problem. (it cured mine)

Yep, mine died at 150,000. But I was lucky in that when it died, it DIED. None of that intermittant stuff.

Highly recommend you listen to the gas filler opening for a in the tank fuel pump whining. If not, the in tank pump is shot. you say the tank is low on gas? Then the in tank pump is suspect because the main pump under the car is starved for fuel and will make noise.

Fill the tank up too