225 Dart chugging past 70...

My Dart has been running great, so my wife and I hit the road and drove 150 miles yesterday, and on the way home, as soon as I past 70 mph, it started chugging and losing power. As soon as I slowed down to 65-70, it ran fine.

Later on, after it had cooled off, we took it out on the freeway again. Same deal. So today I messed with it. Here's what I did.

1. Check fuel pressure. ~ 6psi, even when I revved up the engine.
2. Changed fuel pump for a spare, just to rule out volume. No change.
3. Blew main line back to tank. Heard bubbling.
4. Changed my Mopar electronic distributor back to points. No change.
5. Threw on a Carter BBS I have rebuilt in the garage (I run a Holley 1945). No change.

Now, I'm down to thinking that the sock in the tank's plugged up or the coil's breaking down. I didn't try the coil because I got to the point where I was sick of looking at the thing. I don't think it's vapor lock, because it's never done that before, even on hot days, and it did it when it was not even hot.

Anything I'm missing (pun intended)? :)