Speedo Problem - Inconsistent

[69GMCLonghorn]

Ok, long story which I'll try to shorten up, whilst leaving in all the necessary details:

The old speedo was reading slow by 4 MPH, not by a percentage. Regardless of whether you're doing 30 or 60 MPH, it's off by 4 MPH. I was told the reason for this is the speedo was not properly "zero'd" when it was manufactured. The odometer was dead on accurate.

It was about 3x as much as buying a new one to get mine rebuilt. So I bought a new one from LMC. Now here is what's happening: The speedo will randomly read accurately (within 1 MPH per my GPS). BUT! At its own will, it will read slow at 40 MPH by as much as 5 MPH, and in the same run, will read fast by about the same at 70 MPH.

In English, if the speedo reads 40 MPH, I'm doing 45. If it reads about 76 MPH, I'm doing about 70 MPH... But not all the time! I called LMC and to their credit, they had no problem sending me a new one, which I installed. The same exact results are happening with this one.

Now, you are probably thinking what I am thinking: replace the cable. I'm going to do that, but why wouldn't this have been an issue with the old speedo???

Am I just getting crap units from LMC? Or do you think it's as simple as a cable problem, even though it wasn't an issue with the original speedo?

Help a brother out!!