1997 Chevy Tahoe

What brand of plugs - wires did you use? If the timing was not checked and set by a dealers' computer then the timing is probably off. A timing light will NOT set or check the timing on these engines. The timing MUST be set by a tech 1 GM scann tool. And a dealer is the only one who has one. It'll cost around $50 and about one hour of your time to have this done.
BTW--if you installed anyother brand plugs -wires etc. Other than A/C Delco then you are wasting your time. GM has specificaly designed these ignitions to work on only A/C Delco parts -- Maybe that's why they are going down. Keep us posted