chain slippage

Try using it with external power.

When the trainer is being used in self-powered mode it has to initially generate enough power to power itself up before it can start operating normally (the few seconds of no apparent resistance when you start pedalling). If you are pedalling too slowly you may not be generating enough [electrical] power to keep the trainer operating normally and it will start to power cycle and give short periods of no resistance which will feel like slippage.

If the chain is really slipping then that is a problem somewhere in the drive train (poor adjustment, stretched chain, worn/damaged cassette, etc.).

Thank you. The initial setup was incorrect, so much better now. However, it seems that I can only use the cassette from the middle down to the harder gears. All the easier gears till the middle of the cassette are too easy and don't even go to 100 Watts. Is this normal? I have a 105 drivetrain on an Ultegra cassette, all 11 speed. Thanks

What are you using to control the trainer?

Zwift, thanks

Are doing a power based workout (ERG) or riding a simulated course/race?

I don't use Zwift but if you are riding a course does the resistance increase when you are going up grade and decrease when you are going down grade? If not then Zwift is not controlling the trainer for some reason.

Yes, that part works fine, just in Slope mode 

check the "trainer difficulty" setting in Zwift, in default it is 50% but use it at 100% if You want to get "realistic" terrain. but on flat course as Tempus, if it is very crowded, you will be very fast even with very low power (in ERG You should always use small ring front and 5th cog from the top, in SIM mode this Tempus is a typical big ring course. ) with small ring front and middle in the rear it is impossible to reach higher watts for longer duration on flat course like Tempus in SIM mode (but outside in real world neither ) , only for just a few secs until you reach a higher speed and after that the power is decreasing back significally.