Stumbling GTV6 below 3500 rpm

[Gtv6owner@domain.elided]

I have a 85 GTV6 Callaway. It has developed the following characteristics:
1) Starts harder than it did prior to the problem.
2) Idle appears low and uneven.
3) Once warm it has a pronounced stumble and occasional backfire until the 
rpm's exceed 3500. After that point it appears to operate fairly normal but 
with somewhat less power.

I have checked electrical items, throttle linkage, and fuel (as best as I 
could). I am at a loss and am looking for suggestions. The mechanic I use is 
quite a way away so I would have to trailer it to his place if I can't find 
the problem.

Brian Beckham

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