Possible FI problem

1)Is the unit propane or gasoline? What model?
2) have you checked the hydraulic pump relief valve (electric)- those open electrically to allow the cold hydraulic oil back to the tank to have less resistance on the engine when cold. If stuck shut it puts a hell of a drag on the engine especially cold (below operating temp) That model may or maynot have one it is typically on the pump or on the main large valve bank.
3) if Gasoline are you sure the fuel is good? Most pump fuels are only good for 30 days before they start to turn to crap. Filters wont fix that issue. if propane- are you sure your getting enough pressure - regulators go bad.
4) crankshaft pick up sensor goes bad on those gas or propane. I'm not sure if your ins in the block or the distributor.
5) How old is this unit? If propane- diaphragms go bad on the regulators and the unit that replaces the carburetor. when it acts pokey- try spraying brake cleaner into the intake hose before the airfilter (as long as it is not diesel.... I know you said spark plugs but more than one person told me sparkplugs on heavy equipment and ment glow plugs. see if riching up the air / fuel mixture helps with running. You may have a bad air leak.
6) if really new machine its all electronic control. all you can check is the basics like plugs, cap/ rotor, vacuum readings and basic timing.

Good luck.
Joe