Check engine light?

Hello, my 2005 envoys engine light comes on and goes off in a day or too. Its runs good except it idles rough. And now the speedometer is getting stuck so when im going 40 the speedometer is showing 70. I plugged it in to a scanner in and got code p0302 "2nd cyilider misfiring". How serious is this problem I just bought this truck. What are some simple things I can do to asses the problem before I take it in? Thank you hope u can help me