1996 GMC Suburban

My advice would be to have your mechanic use a water solution in a spray bottle and start spraying ignition components (coil, wires, cap, rotor, )
And see if engine reacts, this is usaully my direction of attack when a customer complains of running bad when wet.

Thats not where the problem lies. GMC almost always puts it distributor right up against the fire wall in its trucks and the rain gets under the hood and drips on the cap and wires. Had same problem with 1989 gmc jimmy and everything is brand new-plugs, wires, cap, rotor. Have been told that the distributor itself needs to be changed moisture is getting inside and causing problem.

Unless your missing your rubber weather strip that runs along the top of the fire wall there should be no water getting on the distributer, just because you have new parts doesnt mean there ok, bad new parts happen, as for moisture getting in the distributer, that one has a small vent on the bottom side, alot of times that doesnt get cleaned out during cap and rotor replacement resulting in premature failure, (wet cap), another cause is the little ears on the distributer that the cap bolts/screws through, they crack, not allowing for proper seal of cap also resulting in wet cap and rotors.