If you confirm your cooling system is in proper order, next step would be injectors. Aftermarket injectors are usually not worth the trouble. It usually entails a lot of practice removing and installing the parts because what you think will solve your problem doesn't, so you have to remove them and start over.
Here's where I'd suggest purchasing injectors. They have excellent service, quality and timely shipping.
Oregon Fuel Injection specializes in the repair of diesel fuel injection systems, replacement diesel parts, performance parts, drive-in service, repairs and diagnostics.
oregonfuelinjection.com