[jhv48]

Put the 6000 rpm chip back in. It will protect the engine from over revving once you tune it properly.

Whether you have full throttle or not wont be the cause of the engine missing. Your problem is probably corrosion in the distributor, or old plugs. Start with a simple tune up. Who knows when the previous owner tuned it up. And if it's been sitting for any length of time, the internals of the distributor could easily corrode and cause just what you're experiencing. Pull the cap and take a look. Replace rotor, cap and plugs. Check for corrosion and make sure the advance springs and plates move easily.

That will eliminate the basics. It's usually the simple things that cause the most trouble.