XPS 15 - Insane Performance Drops

I've been really frustrated with my XPS 15 9560 for a LONG time. It's a €2000+ laptop and it's slower than my super old Surface 3 tablet sometimes...

Issues I'm facing:

• When gaming or running GPU intensive programs, the laptop has insane performance drops after a while.
• The GPU2 [Intel HD] randomly and for no reason whatsoever activates with a super high load. I guess it tries to take over from the GPU1 [GTX 1050]. A game that ran smoothly at 100fps before goes down to 10fps.
• When it happens, the GPU1 and CPU have temperatures of 50 to 70°C so there's no overheating. The GPU1 goes down in load once the GPU2 [Intel HD] activates.
• The performance drops are unpredictable. Sometimes it happens after 15 mins, sometimes it takes 2 hours.
• The laptop does not recover as long as the program is running. Once exited, it takes 15 mins until it's useable again (it takes seconds until my mouse and keyboard commands are registered right now....)

Things I've tried:

• Buy a laptop cooler
• Undervolt CPU, GPUs with Throttlestop and MSI Afterburner
• Reapply thermal paste

The undervolting made it a lot better but didn't fix it completely. I've had periods in the past (about a year ago) where everything ran very smoothly - so I know the laptop CAN do it. It just doesn't do it and it's really frustrating. Why does the Intel HD randomly activate in high performance situations ..only to then make the laptop unuseable for 15 mins?

Any ideas what I can do?

Thanks!