Hey, i went Running Today and the Pace jumped constantly. GPS jumped Even at 21minutes per Kilometer. With my forerunner 735xt was everything Fine but i Need a Pace that i can rely on in a Marathon! I used regular gps and now switched to gps.

Today I learned you can put your entire post in the topic title on this forum...

Seriously, I just about always use all+multiband and that's pretty great for (stable) pace. But overall had no issues with GPS only either, for instant pace that would be pretty bad though indeed. Use avg pace per lap if you really need GPS only

I find the instant pace feature to be pretty accurate even on GPS only but it's annoying how it rounds to the nearest 5 seconds instead of being a smoothed more precise pace at the 1 second per mile rate.  I had a GPS watch literally 10 years ago from a company called Globalsat that could do that with that level of precision.  There's no reason that we can't achieve same standard now.   The hardware can support it... The software just needs smart smoothing algorithms to implement it effectively. 

HI. There are some app that call instant pace the 3"avg pace(or 10").

I think you only need some time playing with your watch setting. 

I've ran London Marathon for the last two years, last year wearing a 945 and this year with the 955. Neither were particularly accurate but the 955 was worse than the 945, which was disappointing to say the least. 945 - 26.60 miles, 515ft of elevation, 955 - 26.77 miles, 381ft of elevation. I ended up working pace out myself at the mile markers, which did help pass the time to be honest!

On another note I've also found pace inaccurate at the start of each lap / mile, even if I'm doing a session at a steady pace. It can take around 30 seconds to settle down to the correct pace. 

Can we consider the 955 a design error? I've never seen so many bugs.