1987 Chevy Astro

Transmission problem
1987 Chevy Astro Automatic 100,00 miles
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Recently had the transmission checked because it was low in changing gears -- the mechanic replaced the fluids

today I was driving it and when I got ready to use the reverse shift on the van, it did not work

I drove the van forward - it worked - It then only drove only 30 miles per hour would not go faster
after a while it then went to 50 miles per hour
which is where I was driving the vehile

is this a transmission problem or what do you think