Engine runs erratically

I drove the vehicle and parked it for a few hours and when I started it, it was making a ticking/knocking noise. I immediately turned it off and checked the oil level, which was fine. After it warmed up, it was idling so roughly that it would die if you did not two foot the gas and the brake. The check engine light has been on several years, but now it will flash when you exceed 35 mph. On the highway, the rpm's are up and down and it is like it wont shift into overdrive. Also, every so often, you will smell gas very heavily for a bit, and then it stops. I took it to my parts store and the printout showed injector #5 to be bad. So, I made an appointment to get that fixed and the shop diagnosed it and said cylinder #5 had no compression and it needed to be rebuilt or a new engine. Do you think the cylinder would just lose compression that quickly? Also, it is weird that the #5 keeps appearing. I had just assumed the heavy fuel smell was caused by the bad injector. What do you think?