give us a good account of how it performs, does it start first go and run fine then stall, or do you have to prime the fuel system then it starts and runs ok.. has the issue just started on its own or has any work taken place to the machine before this problem started or has the issue been there all the time?.. i had a simlar problem on a 966FII with a 3306 it had a hole in the fuel line going to the transfer pump, wasnt leaking due to the tank being below the line but it would let the fuel drain back to the tank over night and not start some mornings if it has been parked for 2-3 days other mornings fine, sometimes it would stall during the day and start after a prime then be fine (when it was low on fuel)
another one i had was a D7HXR it would run fine 99% of the time but if you worked it on load for a good 20mins it would pop & miss and die under load, it has been doing it according to the operator for 3-4 years but wasnt a huge issue as it was only pushing loose stuff most of the time, anyway "everyone" checked this that timming, auto advance gear, fuel filters air filters ect, checked the lift pump pressure and it was 0! but beacause the tank was higher than the engine it feed enough fuel to keep it running most of the time unless you has it on load for awhile, new lift pump, 32psi, ran sweet under load all day
like you metioned its a pretty basic system the fitting you metioned leaving the fuel pump on the return line i suspect that should be an orifice (not a full bottle on a 3204) to restrict fuel flow from the lift pump to give you the transfer/lift pump fuel pressure, sometimes people change these out for a normal fitting and you will never get enough pressure and causes all sorts of issues, check your pressure if its fine and theres still a problem there that will tell you theres another issue somewhere else or if the problem is a fuel supply/pressure issue