blinking service engine soon light folowed by tranny slip

while driving my 98 windstar the service engine soon light which is always on then starts to blink ,when blinking starts the a/c loses its cool and the tranny slips. when light quits blinking the van runs good??????
also i get on the gas ,while light blinking,the rpms will go up but we go slower.

Have codes read and post back. Then we have a starting point.

took it to a friends garage and hooked it up to his snap-on computer,and no codes all tests were good.