what do you mean by switching over around turns or going up a steep incline? are you talking about vtec not engaging? if vtec is not engaging, check to see if the check engine light is on. also make sure your speedo is working. you have proper oil pressure since you just did the oil change so its not that. most likely not the solenoid because they dont go bad that often, but cant rule it out completely. let me know a bit more about the car and i will try and help more.