Engine misfire

I own a 1998 Toyota 4-runner with 212,000 kms. The engine is a 3.4 V6. On a recent trip the vehilce all of a sudden started to run on I believe 5 cylenders, and the engine light came on. We limped to our destination.
- I started the vehicle the next day and it ran fine, no misfires at all. On our way back back home it did the same thing ( ourside temperature at this time was -16 Degrees Celcuis).

- the next day if started fine until it got warn and them misfired.

- I took it to a local mechanic and and they replaced all six plugs and ignition wires. They ran a diagnostic and compression was good and no problems detected. Vehicle was running fine.

- the next day the vehicle did the same thing it started fine and ran great for awhile and then started to miss. I went to work and and shut it down. An hour later I took it back to the repair shop and it ran fine all the way there. The shop had it in the garage and they worked at it for 3 hours and ran the engine hot and could not get it to misfire. They checked the coils and they were all fine as well. They determined it may be a problem with an injector. All diagnostic checks showed everything was running fine.

- did notice when the vehicle was misfiring there was "no" smell of raw gas at the tail pipe.

- the vehicle is now missing again after it warms up and the engine light is on.

- before I spend the money on having the injectors done, do you have any ideas what the problem may be.