I hope I got the right forum for this...

I have a 1967 Plymouth Valiant with a 225 slant 6, that I bought [for only $800] knowing it needed work. But the more my mechanic got into it, the more we discovered that the guy we bought it from  jury-rigged the *** out of it to make it run well enough to sell. Because as he put everything back the way it was supposed to be, it wouldn’t run for *** However we do know that the engine is sound and will run. It has only 97,000 original miles and this  car has been very well kept. We did do a compression check and all the cylinders showed 130.

 My mechanic has replaced the points, cap, rotor, plugs, intake/exhaust manifold gasket[s],  PCV valve, all vacuum hoses, air filter, adjusted the timing and put on a new carb [Plus the guy I bought it from said he had recently replaced the distributor too]. No matter what he’s tried, though, it just won’t run right.. We recently also replaced the fuel pump. That made a huge difference, but not enough of one. It still won’t run right once it warms up.

When I say don’t run right, I mean when the car is at normal operating temp it won’t stay running while in gear, and has major hesitation upon acceleration [bogs down really bad]. It’s not just timing either.

He thinks there must be something he has either overlooked or just missed and now needs a second party, who does this at least semi-professionally – and hopefully someone who really knows these old Dodge/Chrysler/Plymouth engines, to double check and find/fix the problem.  

97,000 miles, I hope some one has replaced the timing chain, I suspect it has an excessive amount of play or the plastic cam gear has probably broken up. This will cause the vehicle to behave as you described.  

Say, have you rebuilt the carb?
And the accelerator pump?
Your 67' slant didn't have the silent chain and if the chains wasted [which I'm sure it is]
You loose vacuum signal and power/torque and that sux. One way to find out is to take the belts off & put a ratchet on the crank bolt turning back and forth you should feel a loose spot, chain slack, this is where the crank moves but the cam doesn't.

But make sure it's not a vacuum leak, be it manifold or carburetor, by spaying a lil
W-D40 or even a mist bottle full of water to find the leak. When the leak is found the idle will lower [cause your temporarily clogging it with the spray].  

Actually if the car was built in Canada it did have the silent cam gear, I had a 65 with this problem.  

Sounds like a vacuum leak to me, do the WD-40 or carb cleaner trick. If you come up with nothing check the thermostat for the choke. My 6 was acting like that and a easy way to find out is tie the choke open.