Hitting a wall at 4K

11sec-cuda said
A shorted out aftermarket tach will cause this problem also. It will apear to work fine.

I did just put an Autometer tach in it right before this started too. hmm

second chance dart said
I had a similar problem with my dart. Turns out it was the short piece of rubber tubing that runs from the tank to the metal fuel line was dry rotted and cracked and was sucking more air thru the hose than fuel. Replaced that and my problem went away.

I never have changed those lines out, they are almost 40 years old, I'll check that too.