electrical problem 1982 mb 240d need help please

I am diriving a 1982 240d. I seem to be having some weird electrical problem. My battery will not hold a charge so obviously I would check battery, alternator, and belts. Did all that and came to the conclusion after the battery was tested fine that it must be the alternator. So with only minimal trickling power I replaced the alternator. Upon doing this I bought new belts and made sure they have good tension, but not to much. I also rewired the starter to the battery to the alternator because the wires looked like they were bad. I am still experience not enough power to restart when engine is cold and I have to jump start it almost everytime. The faster I go the less power I get to all the electrical components. As soon as I come to an idle I seem to get full power. I have checked the belts and they are just fine. I am a poor college student trying to fix this myself. If anyone has any suggestions of what to check I would be very grateful.

I also have a 240 D and am starting college but anyway. I also experienced a similar problem.my battery was constnatly being drained and I would have to charge over and over again. I finally traced the problem to my STARTER SOLENOID RELAY. It changed everything cold starts and all.

Hope I can help good Luck.