I'll try to make this quick:
1. Valve guides being tight... take it to a local shop and ask them to check the clearance while you watch. Even if a clean stem & guide move freely, there may still be insufficient clearance.

There will actually be a little bit of wobble when the clearance is around .002". Once you've felt them set up properly, it's easier to feel when they're too tight... and my last couple sets of Edelbrocks were definitely too tight.

2. Valve job... many aftermarket heads simply don't have a great valve job. In some cases the equipment used doesn't result in a concentric seat, and/or it can be that the out-of-the-box (OOB) valve job doesn't take advantage of more performance-oriented angles & widths.

Edelbrock's OOB valve jobs on Performer RPM & Victor heads are far from ideal in that they have really wide seats and simple angle combinations compared to what a well prepped set of performance heads will have. That results in lost performance that simply lapping the valves in isn't going to recover.

I hope this helps.