By the way just what are you planning on riding up with a 34/28 .
Turning pedals on a stand puts no strain through the drivetrain so won't show things up too well. If you have the right components and set it up correctly then it should work perfectly on the stand and the road though.
Problems could be:
Other components have worn.
Missed a spacer or got them in the wrong order on the cassette.
Wrong length cage on the rear mech for the gearing you've now fitted.
Wrong length chain.
Indexing not set up correctly.
Apart from that it should be fine .
I think Elliot is right and you need a longer cage to run that gear combo. With such big differences in gears you need a longer cage to take up the chain slack correctly. As a set up I would have thought you could have issues anyway as asking it to cope with a range from 34/28 to 50/11 smoothly is asking a lot. There'll be big jumps in ratios (so won't ride too smooth) and the rear mech is going to have a lot of work to do sorting out the chain slack/strain. Normally if you want a big gear spread like that you'd run a triple with a closer ratio cassette, it would run much smoother throughout the whole range of gear ratios.