matching tempos of tracks

I have been trying for an hour to synchronize 2 passes recorded from a 4 track Portastudio with stereo outs. I try to adjust the 2nd pair of tracks to the first 2 by stretching or squeezing parts which drift.
The tempo detector doesn’t work correctly even though there is a repetitive drum part with strong beats.

Then I try the beat calculator and start off OK matching for 4 measures. Then it starts to drift and I do some more measures, After a couple of times, it suddenly upsets all the previous work by resetting to the track start instead of the selection start, and I have to start all over.
There is no undo for this, as in previous versions of Cubase. These bar changes do not show in the history log. All I can do is save after every successful calculation and revert to save when it screws it all up. But it keeps screwing up so I can never finish.

you didnt say if the 4 track Portastudio was digital or a tape machine .if it is a tape recorder you need to export all four track at the same time into the computer, if the porta studio hasnt got four outputs it won`t work right cause tape machines are too unstable to run exactly the same speed twice .hence you will never line up too passes of importing the same song into the computer … if it is a digital porta studio you should be able too line the tracks up easily.

there is no real way (automated) afaik to do this in Cubase… Maybe with certain plugin or perhaps play each track through a Traktor might help tighten things up and reprint to a exact tempo.