Acceleration problems

Hi guys, I have an 82 280Zx turbo that I having trouble accelerating with. When in nuetral it idles a little rough, but revs up nicely. The trouble starts when I try to go faster that 18mph. It starts out fine, but at just below 2k rpms it hesitates and drops down to idle rmps and looses power. This happens everytime like clockwork. I firs experienced this problem about a month ago after a car wash, but after sitting overnight it was gone. Then a couple weeks ago the top radiator hose and alternator belt blew on the highway. I replaced all belts and hoses and now am experiencing the problem above. I recently went on vacation and let the car sit for a week hoping that the problem was moisture related, but it is still happening. I would appreciate any advice on where to start looking? Also the transmission is an automatic that I had tuned about 2 months ago, but it has run well since then. Thanks -Alex