A cracked head, block or bad gasket can let coolant into the oil and cylinder(s) without showing externally.
A worn distributor shaft can indeed make timing jump around and cause the engine to run poorly. So first thing I'd do is check it with a timing light.
Timing should be TDC at low idle, advancing gradually to 35 - 40 deg BTC at high idle. If timing does not advance as RPMs come up the distributor centrifugal advance is stuck and needs attention. Or if the timing mark "dances" (randomly jumps around) the distributor bushings/shaft are worn - time for a rebuild.