Things to check: battery failing, starter not turning engine fast enough, fuel clogged at filter screen in fuel tank (debris, algae, etc.), fuel line kinked/clogged, compression low, injection pump start of delivery is off, IP element sealing washers leak, faulty injection nozzles, fuel pump pressure not high enough, fuel pump check valves leak and/or springs weak, IP fuel return check valve spring has collapsed (common, stretch to 25-27 mm), air/water/dirt in fuel system, bad fuel - this list should give you an idea what to look for.

Good Luck!
Tom

__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.