i have a 88 SSE.just sliips at about 45-50mph..before shifting into OD

Could try flushing & refilling the trans and adding a can of Trans-X but GM had reasons that they made changes to that trans every year. The remanufactured transmissions from GM (for 88 SSE with code "C" 3800 and 2.73 ratio FP3 trans the p/n is 24212182 ) have all of the updates.

When was your last ATF change?

last ATF change...

sorry for the nubness..tahts a new acronym for me

ATF=Automatic Transmission Fluid

Also, when did you get this "New" Tranny installed?

Oh yeah, Im going to change the fluid and the filter,

as I just found out that the guy who owned the car before me put in Dextron III

A lot of people pronounce/spell the oil that way. Actually the transmission oil is called DexRON and you should use the same. You will probably see it on the shelf as Dexron III/Mercon.

arent you supposed to use Dexron II in the Bonnies?

Dexron III is backwards compatible. It works for anything that used Dexron II

Dexron III has been known to kill the Toyota/Aisin (Aisin A340E) slushboxes designed for Dexron II in the Supra/Cressida. Apparently it'* too grippy for the clutches and overheats them. Folks have fried their trannies after only 1500km of Dexron III, and I know of another guy whose reverse just isn't the same anymore, luckily he caught his error soon enough.