Won't acelerate while going up hill on 1997 Pontiac Bonneville

My 1997 Bonneville won't acelerate going up hill. The RPMs go up, the spedominer goes up, the engine revs up and the transmition sounds like it's chainging gears as normal yet it goes very slowly up a low grade hill, (apx 30 mph). This occured apx 3 months ago. The next day it drove perfectly normal. The engine, or any other lights have never come on during this eppiside. Please Help

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1 answer 3 comments
No "check engine light" or any other dash light displays. No codes either. It seemed like I was stuck in gear (or neutral) but yet I heard and felt it change gears as normal. A MAC truck almost plowed me over and there was nothing that I could do. I didn't have any pick up. Once coming over the incline, she ran top notch. Great pick up, rides smooth as glass and gears change effortlessly and un-noticeably. Please help me...Thanks
possibly a restricted exh system, like a partially plugged cat
Thanks so much