l have an 03 Mercury Sable 3.0 Duratech engine. The other day (the coldest day l drove my car since l bought it used) started to rumble/shake as l pulled out of my drive way. Once l got her on the highway, it was all fine. Today l drove about 30 miles to school, everything was fine. During lunch l was trying to accelerate and pass a guy on a bicycle when the car seemed to no accelerate at all, as if it loss 50% of its power. At this point l was thinking it had something to do with the transmission. Anyway when l finally get out of school and head back my 30 miles it was running good until l got on the on ramp to the highway. As I am merging on there was another on ramp up ahead with someone else merging on, so l got over and started to accelerate. As l got up to 60mph the car started to shake and rumble violently, rpms shot all the way up then dropped to 3k rpm. The car accelerated in speed very very slowly and a light started to flash on the gauge cluster. l looked it up and it was an emissions light. Manual says if its blinking it may be a misfire code and to not accelerate heavily or else the cat might get screwed up. Afterward l drove it very calmly and took it easy. The car did not have a problem until l started to get up and around 60mph or if l was going up hill. Has anyone ever have this happen to them? Or would anyone have any advice/pointers? Could it be as easy as dumping in some fuel injector cleaners into the gas? Could it actually be the transmission instead?? Thanks for any help in advance. Thank you! Please check back if you ask me a question because I will respond shortly!

First, check that the transmission fluid is full, where it should be on the dipstick. If that checks out as ok, then it is probably what happened to my sable, the fuel injectors are clogged. First try some chevron teck carb cleaner and see if that clears them up. If not you will probably have to replace the injectors to bring the car back to life.
Good luck.

Take the car to auto zone and have them pull the computer codes for you and this will tell you what is worry. They do this for free.

Thanks for telling us what engine it has. Rule of thumb: always start with basic maintenance before thinking the worst. Are the spark plugs good? Are any of them clogged with oil, or appear different? Have the timing belts been replaced according to schedule? Has the fuel filter been changed? If the basic maintenance is covered, & there is no check engine light, there could be a clogged EGR valve, or maybe the EGR valve does not open up all the way. There might be a vacuum leak. If the engine is running very rough, open the hood, & with the engine running, listen closely for a ''hissing'' sound.