ahhh the ole setting the timing by ear method...
yes i used to do this in the old days when i was trying to power balance a truck :oD
of course now a days with the onboard ECM's this generally doesn't work so well and can cause more problems than helping.
If you want the timing set to the correct range you will have to do it the factory way.
According to the RG manual the timing for the FE and F2 engines are to be set to 0°deg at 725 rpm's on white mark on gasoline engines, and 9° deg for LPG engines, at 725 rpm's using the red mark.
when installing the belt the crank should have the deflector washer knotch pointing straight up at 12 oclock aligned with the mark on the engine, the crank key should also be inline with this knotch, and the cam gear mark should be set with the letter A on the gear pointing at 12 oclock aligned with the mark/knotch on the engine.
The part that usually causes problems is when you set the tensioner, that may move either the crank or cam causing the marks to go out of alignment. This is why you always rotate the engine after setting the belt and tensioner to insure the marks are still where they should be.
IMPCO fuel system idle speed: 725 rpm
AISAN fuel system idle speed: 800 rpm
Also as was suggested you may want to recheck your timing mark locations and make sure they are on the mark, alot of times when you install a belt on a multicog setup like whats in these engines it is not hard to be a tooth off, it happens to me quite more than not so it is a good practice to rotate the engine a couple of rotations after installing the belt to let the belt settle in place and allow you to see if the marks are indeed on the mark.
Being one tooth off can make all the difference in the world and can cause a severe loss of power and erratic running performance.
Recheck the cam timing. You might have the belt off a tooth.
What was the reson for the belt change,did it break??? If it did is it possible a valve is bent and hanging open just a bit?.....is this the same truck with serial number that starts with A875 that you posted about clutch inching issue???