84 528e Tech question, I need help please

Hi, I have an '84 528E with a problem I cant figure out. When I start it cold in the morning, it runs fine. As soon as it warms up for a minute, the temp guage on the instrament panel goes dead. The motor then idles irradically. If I give it alot of gas, it backfires in the exhaust manifold like its loading up or flooding. It also blows out black smoke from the tail pipe. I checked the sensors, they work fine. I replaced the motor, and it has good compression, but the same problem. I replaced both fuel filters and checked the fuel pressure. The fuel pressure is fine. Sometimes when I drive it cold the temp guage works. Then my electrical problem happens and the temp guage works, but the light on the instrament panel that tells you when its time for service goes out. ( its the one that is green, then goes yellow, then red.) I have been going out of my mind tring to find what the problem is and I need a pro, or someone who has had this problem happen and knows the solution. Please help me!!! I love my BMW and want it to run good again. Any sugguestions would be helpful.

THANK YOU!!!!