I have a 68 coupe with a 289 2v with factory A/C. When I am on the freeway and the car is hot ( I live in AZ), the engine begins to miss at 60 mph. If I continue to try to accelerate, the car will die. However, If I drop the speed back down, I can limp it home. The temp gauge doesn't read overheating but when I turn the engine off, the over flow begins to weap.
Anyone have any suggestions?
Mike  

I had this same thing going on in my 68 last summer. The vacuum advance seems to be the issue. with the car off use a vacuum pump at the distributer take the cap off and see if its operating or that the diaphragm is holding the vacuum. mine would advance but would slowly leak and the arm would sneak back. The excess unburned fuel mix would heat the heads up at turn off.  

Hey, Thanks for the quick response. I put a vacuum pump on the diaphragm and pumped it up. The advance did move and the diaphragm held. I also did a vacuum test on the engine and it checks out. I disconnected the vacuum resevoir but that didn't help. It only begins missing as the engine heats up and gets worse the hotter it gets.
Do you think that it might be condensor? or maybe a radiator problem? It seems to relly get hot once the engine is shut off.
Again, thanks for the igreat input.  

I messed with the points in my 302 almost monthly for around two years. I went to a electronic ign. all my problems with hard starts and engine miss went away. If you are into keeping it stock looking I would replace the points with petronix. It all hides under the stock cap and is simple to install. What is the temp. at if its high I would flush the cooling system and put in a 180 thermostat. my small blocks from the past all seem to have loved running cold the gas mileage will suffer but who cares it sucks already anyway. I have seen radiators for 125 at laurel mountain mustang is yours old? I would give it a good flushing and then try testing again I would also replace the fuel filter and the condenser. cause they are cheap and easy rule outs. Let me know what happens next.- Ron