Having problems with my Chevy Blazer starting

I have a 1993 S10 Blazer 4.3 l 4x4 with approximately 125,000 miles, And the truck will trun over but wil lnot start. We have replaced the relay, the fule pump and the fuel filter, and our mechanic has even replaced the gas tank, because it was all rotted out. Today he change the fuel injectors, thinking that was the reason why the vehicle is not starting however that still is not the problem, he says that there is plenty of fuel pressure behind it and that there is know no reason for it not to start. I will turn over but will not start. We have replaced the battery also. As you can tell this truck needs alot of work, we reinheriated back from someone in the family. Any suggestions on what we can have our mechanic check. We and him are at a loss as to what could be wrong with it, Oh yeah also if he sprays ether on the components it starts and runs until it burns off then it dies again and will not restart. And also he has noticed that heater core is shot on it too, Please any suggestions as to what to check and what to do would be great. Thanks