Need help with '85 maxima

The problem is that while I am in first gear my car stops accelerating at 15mph and then gains speed very slowly. Once it gets into second gear (25mph)the rpms jump up high and then it works. Also when I was stuck at 15mph I shut off the overdrive and it started accelerating somewhat close to normal. If anyone could help me out and tell me what is wrong I would appritiate it.