I'm having some interesting issues with my '76 vette and hoping someone can help!

It is possible that your starter is getting hot.  They make heat blankets to keep them cooler. 

It might be worth driving it around and pulling back in your drive and check the temp of the starter.  Way too hot to touch is way too hot.  It should be hot but not header hot.

It could be a loose/corroded connection at the starter solenid, or even a bad ign. switch. I have also seen corroded terminals at the firewall pass-thru(at the fuse box) that will cause enough of a voltage drop to not feed the "start" terminal of the solenoid properly. Hooking it up to another vehicle to "boost" it off will supply enough voltage to start it.

Check the connections at the starter first, and make sure the ground cable is good at the frame under the batt. box, and from the frame to the engine at the starter area.
If you have a volt meter, hook it up to the purple wire(closest wire to the engine side of the solenoid) on the solenoid, and check the voltage while cranking. It should be at least 12v. This will help determine if the start wire is getting full voltage. If it is not getting 12v when the car doesn't want to start, the problem is in the wiring/connections from the ign. switch.

I'd be looking for a bad earth as Joel says.  If there's grunge in the joint it can create a high resistance as it gets hotter. 

 do you have headers on the car...,,if so id say the starter is getting too hot,,if not, you  either have a bad terminal on the wiring or you have a ford gemlin

If the car is turning over it most likely is not the battery...

If it does NOTHING after a 15 minute drive I would suspect heat sink problems... drive it 30 minutes and RETURN home... shut it down for 3-4 minutes and try to start it.. IF nothing happens pour a quart of water directly on the starter...of it starts...ya got the old heat sink problem...

Ground... ?? clean the connections...especially the rear frame to battery connection..is you still have the problem  ( cable over 30 years just may be corroded INSIDE...been there done that...   Just take a jumper cable and connect the neg post of the battery to someplace on your frame...now you know you have a good ground ....

Bob G.

I'm interested to hear how this turns out, please keep us posted on your progress with tracking this problem down!  I have had this problem occassionally, and I think it is a heat sink issue with the headers and starter.  I just haven't gotten the heat shield blanket yet.  I will be looking for what you find out about yours.

My 76 never has been bitten by the heat sink problem...BUT both my 72 and 79 suffered from it ...

I solved the problem years ago very cheaply on the advice given me by a young lady who was the crew chief (yea and she knew her stuff) on her husbands sprint  car... 

She recommended that I replace the stock solenoid spring with a AC DElco GM spring Number 1958679   ...  I did that maybe 10-15 years ago and I  have never had the problem again...

This spring felt and looked the same as the stock spring but was not nearly as strong as the stock spring...thus it required much less electrical energy to compress ..  

I was so Happy that I replaced the Solenoid spring in all 3 of my C3's and my Chevelle and to be honest none of the cars ever had the hot start problem again...

Unfortunately I think that GM has discontinued the spring...at least that is
what I have heard...   Darn think only cost about 5 bucks... 

Maybe this spring is available from another source now I do not know but
from what I understand The Sprint car and Modified Racers almost to a man (or gal) ran this spring all the time...

Bob G.

 

My problem was caused by an old corroded battery cable to the starter. I eventually discovered the problem when the cable came loose enough to make contact with the frame causing a fire. No serious damage done but scared the crap out of me.

As a added thought to Bob's post, there is also another neat "trick" to help with hot start issues, that we discovered back in the day on the race cars.
You can install a(please excuse my language here) Ford starter solenoid on the car, under the hood, and with just a couple of minor wiring mods, completely solve the GM starter solenoid issues. I've done this on countless race cars, boats, and many service trucks w/big-blocks. Works EVERY time. It won't solve any loose connection issues, or make a bad starter become new again, but it will eliminate the cheesey solenoid as a source of no start problems.