Puttering, drops in power weakened idle?

I have a 1985 Nissan Pickup and on my way to work this morning it began to putter and when starting out in any gear, but mostly first, it would require me to highly rev my engine. The idle dropped from its normal 800-900 to 400-500 RPM. It stalled once. It is carburated and in June I experienced a similar problem, it turned out to be my carburetor and i had it replaced along with my fuel filter, fuel pump, spark plug wires, and spark plugs. What do you think might be the issue?