problem with transmission while driving
 

hello!
blazer k5 1990, 5.7L - while cruising on 90-100kph the transmission seems like it can't decide in which gear it wants to be in.
this "jump" happens several times in each drive, while the gas paddle is in solid position of course. a quick jump between the third and the forth and then it settles down.
any ideas?

Your transmission has a lock-up torque converter. It sounds like that is going in and out of lock up.

Do you have a tachometer? If so, how much does the RPM rise/fall during the event? What RPM are you running if you pull it down into 3rd gear?

no tachometer and no rpm gauge of course..

sounds like your 700r4 isn't holding the lock up. there is a way to make a manual switched lock up.