another transmission issue and bogging

so for a while not my gears shifting into second and sometimes third at higher rpm's grinds?? i talked with a mechanic at my work about it and he says it could be the transmission.. anyone else half to replace theirs? also for about a month now my car has been bogging slightly but enough to annoy me wuit a bit. my car seems slower also. my cel has been on also and i hooked it up to the comp. and it didnt come up as anything. i was thinking the mafs or maybe an o2 sensor. im going to be putting my car up for the winter soon and wanted to figure this out soon so i can get it fixed during the winter.. any helps appreciated