Help!

[r2me24]

I have a 98 GMC Sierra with 125k miles on it. the radio started to just turn off and then back on, and then my gauges would jump a little too. Then it got worse, The entire engine died, it would start back up, but would die if I was driving about 30 mph or slower. I had to throw it neutral and raise the rpms while turning just to keep it running. Checked battery voltage, tested alternator and all is good, I ran a scanner but didnt get any codes. I just started it up and heard a huge bang and saw smoke coming from the engine. I tried cranking it again and started right up, but check engine light is now on. I put a scanner on it, and it tells me that cylinder 5 is misfiring. Could this be the root of all my problems???? Im curious as to why the engine light just now came on, and why the scanner didnt pick this up before. can someone please help????? Im gettin desperate.