Disconnect power to distributor, take valve covers off, use remote start button or have friend hold the start button, and watch each one of the rocker arms in action. (take spark plugs out so motor turns over faster and easier on the starter) Check slop on any and all of the rockers with both valves closed on compression stroke as soon as intake valve is completely closed.
Even if you don't find any loose rockers from exessive lobe wear (if previous owner tightened it down before selling to you), you should be able to see if one or more are not travelling as much as others. If you have access to dial indicator and magnetic holder better yet to actually measure valve travel without having to tear motor apart to pull cam to see.
I also agree possibly a bad timing chain issue... but of course make sure your distributor rotor is right on with #1 cylinder with both valves closed on the compression stroke. Watch the intake valve closely when lining up your harmonic balancer mark to the pointer to make sure you are not 180* out of sync but actually on the compression stroke. With spark plugs out you should be able to turn crankshaft by hand to get it right on to #1.