Very Involved Problem 400sbc
Read the article Cobalt327 provided a link for. To me it sounds like it is advance related. You say you changed springs in the distributor they might be too weak. Most of the time you can use 1 light spring and a medium spring. Get a Crane PN #99600-1 vacuum advance kit. Start with two blue springs. Unplug the vacuum line from the distributor and plug it. When checking, have someone hold the rpms at 3000-3500. Set the advance on the timing light to 36* and the mark on the balancer should align to zero. Then zero the timing light advance. Check the timing at idle. You will want your advance curve to start at about 800 rpms and to be all in by 3000 rpms. Make sure that the weights are not sticking, clean and polish them if necessary.