Computer Problems

I have a 1997 gmc safari and it keeps on burning the computer. I have replaced it 3 times already. Every time I replace the computer, It runs for 25 miles or as much as 400 miles before the check engine light comes on and/or the speedometer quits working. some times the computer code points to the transmission sensor, some other times it just burns the computer. The smell of the computer burning is almost like gasoline. and after I pull the computer out; a nasty liquid pours out of it. I have replaced transmission sensors and related sensors without success. Can anyone provide an idea as to what may be wrong with this Van?