Z31 doesn't go over 25mph

[MarineVet]

After driving for awhile my transmission started slipping. I thought it was low on fluid so I put a quart in it. Then after driving for awhile every time I went over 25 mph it started to sputter and wouldn't go over 25mph. I noticed I couldn't put it in overdrive when this happen. The transmission was rebuilt last year so I'm totally clueless on what the problem is. Can anybody tell me what's wrong?

[evildky]

For clarification, if you put the trans in neutral you can rev up to red line? if so, whatever it is it's in the trans and or torque convertor.

[MarineVet]

Yes if it put it in neutral I can rev it to redline. It only starts slipping when I drive it for more than an hour. But the thing is sometimes it won't go to redline when the overdrive light doesn't come on.

[G-E]

Try a fluid/flush/filter change on it first... it may behave wonky for a week after but it should clear up

If it's still slipping, either you need a rebuilt/new valvebody because it's too gummed up, or you need a full rebuild to tighten up bands and such... unfortunately that can mean spending money on fluid more than once as you go for deeper service, if any previous step didn't fix it :\