ls-olds

Swapped an 8.1L engine into my 84 Dually to pull the Cutlass around.(same fuel system as ls engines) Did the swap last fall and been running fine until the last couple of weeks. Has injection tanks(new) swapped in with two walbro intake pumps running thru a switching valve. Currently running a vette filter/reg just ahead of the valve. Single line going to the front(plugged return at the fuel rail) 58psi at the rail. Starts up and runs fine in the morning. After warming up and running for a awhile if I stop and shut off the engine it will stumble(pretty bad) when taking off. Starts right up all the time(hot or cold) Will hold fuel pressure when shut off(might drop 10psi in an hr). Acts like it is flooding but no black smoke to indicate that. Runs fine after a few seconds. Ideas?