I have frequent misfires, and sometimes a loss of power that makes it hard to maintain speed at or above ~60mph. The only code I get is a misfire code. Happens only in 5th gear, at any speed, RPM or throttle position. I switched from copper to iridium plugs, replaced spark plug wires, tripe checked plug gap. I can't figure out why it would only happen in 5th gear. Car is about 101k miles, all stock.

 

check your coil packs

 

Consider this. Running in 5th gear & imputing a high load into the engine (lugging the motor) puts a lot of stress into the system

Are you getting any CEL's

Sounds to me like you should get the engine logged / tuned to see whats going on