Just plain wierd

Here's the full situation. A couple of months ago my cat conv. became plugged and was subsequently replaced. Less than 2 weeks after replacing it, I got a check engine light and the engine started to run hard. I scanned it and got a #5 cylinder misfire. Okay, no problem. I checked the wire- it was good. I pulled the plug- some carbon build up so I replaced it. I cleared codes and started the vehicle and got the engine light again with a rough engine. Scanned it- #6 cylinder misfire. Checked wire- it was good. Checked plug- it was good. Resaeted all connections and cleared codes. Started vehicle and got the same thing. Scanned codes and got a #5 cylinder misfire. Back and forth I went with no joy. In the middle of this I also changed the cap and rotor just to eliminate them. I suspect a bad ignition module, am I right? But wait, there's more. Now, when I turn the ignition to on, I get minimal voltage read from the battery (fully charged off the charger) and after the vehicle starts running, for the first 5 seconds or so there are absolutely no indications on the gauge cluster and no interior lights. All the sudden everything comes online with all gauges functional. I suspect a bad ECM or BCM, am I right? Please help, I don't feel like paying a mechanic for something I can do myself.
I am working with a 98 Blazer with a V6 Vortec.

try checking your ground wire and positive wire coming off the battery or junction box, also see if you have any fuseable links that may be corroded or loose connections.