1986 BMW 3 series; unable to prevent radical engine pulse.

1986 BMW 3 series; unable to prevent radical and/or up and down engine pulse. I've installed starter and fuel pump also turned distrbutor to keep it running. From a cold start it seems not to breath enough to stay running so I give it more gas to kep it running. When driving the engine goes to so many rpm's and then no more until its in third gear and going at a certain speed. I have a broken muffler and I believe I need to replace my catalytic converter. "What should I do".