1500 bucking problem

I have a 1991 Chevy 1550 with a 4.3 6 in it,lately the truck has developed a bucking at low RPM and under a load,if i stomp on the accellerator while at about 40 mpg or less it will back fire through the carb,i have only changed the plugs in this vehicle(last week) and fuel
filter.
Some days it won't do it at all.
I do not get a "check engine light".
The truck has 41,000 original miles.
What could it be?
It also seems to do it on rainy days more than others.
Any comments would be helpful!

I would suspect the plug wires to begin with. Though an o'scope would help, the plug wires would be the place to start. Ray

My truck was missing out because i had a dirty rotor cap,i replaced the cap and rotor and she runs nice!
I had replaced the plugs first,that didn't do it then took off the rotor cap and it had lots of corrosion on the points inside.