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Not sure if chain or derailleur problem
Just put a new chain on, same size, on MTB and rear drailleur pulleys. Now when running through gears on stand i have no problems, but when riding the rear derailleur will pull toward the front and bounce back. It doesn't cause clicking or gear change, just seems to pull derailluer forward then in bounces back. This occurs in all gears. Stumped on what could be causing this to occur.

Shimano XT derailluer with Shimano pulleys

SCRAM PC991 chain
Might be a stiff chain link from where you connected it. I had this problem on my bike. I just flipped it over and slowly rotated the chain through the rear derailleur and if I saw any sticking I would loosen it up.
Does it do it continuously or once per chain rev ?
If continuously then maybe wear on the chainrings.

Is there any effect if you turn the chainset backwards? (does the derailleur jump?)

Does it do it on all three chainrings ?

It's possibly "chainsuck" which means that the chain sticks to the teeth of the chainring. (new chain & old rings)
As the chain goes round it sticks to the chainring which causes the derailleur to pull forward.
That is until there is enough tension in the derailleur to pull the chain off the ring letting the derailleur spring back.

With the chain being new it's possible that its a bit stiff with grease and this will have the same effect but should wear off pretty quickly.

The obvious difference, between checking it on the stand and riding, is the force on the chain and the drag of the wheel.

Just check that neither of the jockey wheels is sticking but I can't imagine this would cause the symptom anyway.
The only other thing I could think of would be a sticking freehub but this would show up on the stand as well.
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