Engine starts good, then other times just turns over, Why?

1997 gmc safari, 187 thous kilms, Motor runs perfect when running, then next time try start will just crank over, have changed Rotor, coil, crank sensor, has plenty of gas pressure got spark but not strong, although this stayed same even after changing rotor, coil and crank sensor, hasent started for 5 days now but maybe 2 morrow she will, had this problem on and off fo several months. When running you can turn van on and off plenty of times no problem then after a while minuts or hours just crank over, I have heard alot and tried alot any thing would be appreciated.


Ed