If you think some problems were caused by adding the amp, try adding a small attenuator to the input, the output, or both. This hardens the amp against any impedance variations. A terminator is nothing more than an F connector for RG-59 coax with a 75 ohm resistor soldered in the sleeve, the free end serving as the center pin of the connector. You can buy them premade, or even make them yourself. The Terminator Unterminated splitter ports can cause reflections on the coax.
Reflections generally show up as ghosting in the picture on some or all channels.
Terminating these unused splitter ports keeps the reflections from occurring. If the engineering is a little over your head, companies like Channel Plus make prengineered systems. These can be simple distribution systems, or include modulators. With a little thought and experimentation, even those of us without a cable tv connection can build something similar at home, and be able to use the cable ready features available on newer TVs and VCRs. I don't really expect that any one will exactly duplicate our home cable system.