88 Coupe Deville Trans Problem
 

While driving the trans will shift through all the gears normaly until it shifts into its highest gear, then it drops to the next lower gear then shifts up again then back down. It does this a few times then stays in the lower gear. I got this car from my mother who had this problem once and took it to the dealership and cost her about a grand$. Said something about an adjustment? Any suggestions? Thanks in advance!!