93 sierra, code 32, but only when it rains

I have a 1993 Sierra with a 4.3L 6. It has 170,000 miles on it. It runs great! As long as it doesn't rain. When the weather is wet, it missfires at lower RPM, but accelerates well when the revs get above about 3000. the engine light comes on and I get a code 32, but again, only when it rains.

Cruising at 60, when it's dry, it normally produces about 12 inches ofr vacuum. When it's wet, it's between 5 and 10. The problem seems to get better after about 15 to 20 miles of driving.

I have replaced:

The EGR valve (problem got better when dry, but still runs bad when wet)
EGR solenoid (again, ran better, but wet problem still not solved)
vacuum lines (made it better still, but continues to run crappy when wet
Ignition wires (helped even more, but again, runs bad when it rains).

I looked through the service manual, and it mentioned that misfire on acceleration may be due to a missing preheater hose. It is missing, but I doubt this is the problem. Please help before I torch it.