Ford Repair: Coolant

Question
QUESTION: Great!  You've been working on Fords for 30 years; my 1986 Mustang is 28.5 years old.
1986 Ford Mustang GT, 5.0L, 5-speed manual trans., A/C, 481,000 miles (seriously), no mechanical engine work ever!
So... yesterday was really hot, approx. 100 degrees here in Orange Co. CA, and my Mustang was running hot - the temp. gauge was above normal, although, not outside the "safe?" range.  I parked at the Health Foods store, went in for a half hour, and when I came out, discovered a long stream of antifreeze running from my car.  It's done this before in really hot weather, once I saw it happen - I suspect that when the engine is really hot when I turn it off, the heat from the block causes the antifreeze to overheat or boil and then it escapes out through the overflow tank.  The problem does seem to gradually be getting worse... it used to only do this when I was driving in the mtns. on a hot day.  (O.C. is mostly flat.)
Anyway, I'm just wondering if I need a new radiator?  It seems like it's not keeping the engine cool enough to prevent these "explosions" - and man, that hot water comes exploding out of the overflow tank.  
I have a small coolant leak in the vicinity of the timing chain cover, but that seems to only leak when the engine is cooling off.  The cooling system pressurizes just fine, and all the hoses and connections are good.  (I just have to add a bit of coolant periodically.)  I have pressure tested the system when it's cool, and the timing chain cover leak is the only one I've seen.  And it minor; probably not involved in the "explosion" problem.
I'm on my 3rd or 4th radiator, and the tubes look a bit crud-ed up from the little bit I can see, although they don't look seriously plugged.  In the off season (i.e under 100 degrees), I can drive all year without a problem, but this has been occurring more frequently in the summer.
What's your best guess?  I'm thinking I need a new radiator.  But a good one is at least $300, so I want to be reasonably certain first before spending the time and money to replace it.
The fan and fan clutch seem to be working okay.  My A/C also works good - just rebuild it a few years ago.  I'm sure it was adding to the heat load yesterday.  
Thanks in advance for your thoughts!

ANSWER: Hi Brian,
I believe your problem is the water pump. See if you can see under the water pump and if there is coolant coming from the weep hole. Also having a few of the 5.0 myself all of them I had to use cool tech. That is an antifreeze that holds heat and dissipates heat very well. Make sure you flush your system out, which I'm sure you already have done. In the mountains your parametric pressure is higher than at sea level so your combustion actually is higher causing the rise in temp. Good luck and let me know if you have any other questions.
Jeff

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QUESTION: Hi Jeff! Thanks for your reply.  To clarify....
1. Are you saying that you think the water pump is causing the overheating, or that just where the leak might be coming from?  If overheating, please explain further... it IS old, has 100,000+ miles on it.  Do the impellers disintegrate?  It seems to provide plenty of heat in the winter, so I presume the water is still moving through the block okay.  If source of the leak, I don't think so.  The weep hole is visible, and I've never seen any water come out of it.  My (former) mechanic crawled under the car and said the drip appeared to be coming out of the block near the timing chain cover.  At first we thought it might be the  connection to the lower radiator hose, but it's lower than that.  
2. Does coolant flow through the timing chain cover on 5.0L engines?  His speculation is that the cover might be corroded, or a gasket might be failing.   
3. "Cool Tech" antifreeze?  I don't get any hits on that on Google... where do you get it?
4. "parametric"?  Did you mean barometic pressure?  I think it's lower in the mtns., not higher.  Regardless, if it makes engines run hotter, that's good to know.  Never heard that before.  Anyway, now my boilovers are occurring at sea level, alas.
-Brian G.

Answer
Sorry Brian,
Yes I meant as far as the water pump goes, the impellers do corrode away. And yes I meant barametric pressure. Spell correct. Yes this will affect the engine on vehicles and motorcycles. I will find out another name for the antifreeze. Cool tech is just the brand I used. I know He's catalog had it.
Thank you.
Jeff