Engine surge The emgine surges forward on my 2000C70 Volvo

You will need to replace your throttle body.

No, remove and clean or replace the IAC valve...use trottle body cleaner or brake cleaner..IAC means Idle Air Control...cheers

The power brake booster runs on engine vacuum to provide the power assist. Since it surges at idle and you mention that hard brake pedal application causes the surge as well, along with the knowledge that the engine was recently replaced, I'd be looking for a vacuum leak somewhere.

If you had the engine installed by a reputable shop, you should take it back to them assuming it started doing this just after the engine was installed. If they have a smoke machine for locating vacuum leaks, that should make the search even easier.

on google:

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ON THE UNDERSIDE OF THE INTAKE MANIFOLD THERE ISA PART THAT CONTROLS THE VOLUME OF AIR FLOWING THRU THE PCV SYSTEM. If is ruptured you will experience surging and many other problems related. To mixture. THIS MODEL HAS PROBLEMS with the electronic throttle control in the throttle body. Neither of these are easy fixes. If you remove thedipstick whith the engine idling and the engine begins to runrough it pcv related. If when you experience the surging you move the gear selector from D to 3 andte surging stops it is relatedto the throttle.

Replace fuel filter and check for fuel leaks. Check and change the pollution canister or purge valve.

have u checked the idel air control selonoid otherwise could be a bad injector or fuel pressure regulator

You could have a bad or contaminated Mass Air Flow (MAF) sensor. You can get a used one online for less then 100. You can also purchase a can of MAF sensor cleaner at a local auto supplies store and clean it. It located right at the air filter housing.

it could be a intake leak or low fuel pressure.

Sounds like your Electronic Throttle System Software needs replacing. Contact an authorized Volvo dealer shop.
There's a recall so it will be no charge.