Oh yes, the ever so fun derailleur fix

I bought a older 10 speed and I'm trying to set the gearing but my rear derailleur is popping.

Here is the scenario:
On bike, going down road.
When i peddle hard, to push the bike to go faster, the shin pops and I get a small jolt
I change gears to second, nothing happens, no pop
I change back to first, pops when I push hard

I've tightened the mount, played with the limiter screws and adjustments, readjusted cable tension, set shift tension.

Problems: Older derailleur, suntour cyclone rear, higher rear gearing on stock derailleur, and a bad pin in chain that doesn't bend fully - this isn't that bad, just off by an angle after it goes around a crank, but could be the problem

Any ideas? thanks in advance.