i have a 1989 volvo 740gl . this car starts when it wants too. i have change the fuel pump fuel feed pump and fuel filter,put in new battery, new alternater, new starter, and still its doing the same thing? it starts for awhile then it decides not too, but you put jumper cables to it and it will start and runs fine . you shut it off and it may start and it may not??/   please could someone tell me were to go next

Optional Information:
1989 Volvo 740 GL

Already Tried:
new fuel pump, fuel feed pump, fuel filter, new battery, new alternater, new starter, new air filter, new spark plugs, new distributer cap, & bypassed the key switch.