The capacitor is just a temporary fix. It's just more work that the alternator has to do. Your electrical system shouldn't be flickering with 400watts. I had 600 watts on my stock system and the only time it flickered was if I hadnt started the vehicle in awhile or if I used the windows. Once I got a optima yellow top I went to about 1,000 watts and I still didn't have flickering. Every GM truck I've been in has flickered when the windows are being rolled down or up. I would sell the capacitor and get a better battery from like optima or something similar. I would also do the BIG 3. That will probably help the most.