An old post in Volvoforums.com says 510 is for LH 2.1, not your LH 2.0. That makes a bit of sense not being in Bill Garland’s ECU table as LH 2.1 is quite rare and that table was mostly user contributed and not necessarily from Volvo sources. Best to check an authoritative source before ordering.

Not being able to get above 2700 is almost exactly the governed limit for being in limp home mode. If the 507 ECU can’t see things like the AMM and I believe also the O2 sensor then it will go into limp home mode and ignore sensor inputs. If your 507 doesn't see an O2 sensor when it’s expecting one then that might explain your symptoms. If you now pull your AMM connector to force limp home mode and it behaves the same then that would seemingly point the finger to it already being in limp home mode and confirm my theory.

It smells to me like you may really need a 503. If you can’t find one listed anywhere, maybe ask a local yard to reach out on the wrecker network for your year and model being available as a parts car somewhere.

Re. Phil’s comments. I can attest to Injection Labs being a quality source and if you can get their stuff through FCP with their lifetime guarantee then all the better, although you’ll pay a few dollars extra for the privilege. Check the fine print, I just now vaguely recall there may be an exception for Injection Labs stuff.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now