ne other idea is that maybe it a shim is missing on the input tape guide (or output, if the stationary output guide has been replaced by a ball bearing), so that the tape gets pinched and resists traction enough to cause irregular speed - I have seen it happen on occasions. Also, make sure that the spring-loaded guide pin located on the left of the heads moves freely: if jammed it might cause speed fluctuations when the tape reaches the end (case seen on the pneumatically damped guide pin of a PR99)..
Another thing to check when you hear W&F on these decks is whether the capstan itself has been polished smooth by use. Overly smooth capstans can be given a new life with a bit of blasting.No wonder that you mostly notice it on piano, it is the most sensitive instrument to W&F. Personally I hear it when the value reaches 0.1%, but the threshold varies from person to person. The usual suspects are the condition of the pinch roller, the pressure of the pinch roller (it has to be 1.3 kg +/- 100 g, and experience taught me that this is critical), the clealiness of the capstan (any crud on it?). It may also be caused by dragging brakes (sometimes the brakes remain very slightly engaged during play), or (quite often) by decaying tape tape, especially (but not only) if it sheds oxide on the tape transport, so you should make your tests with fresh tape.As a general rule, check that the whole tape transport is free from crud and debris before and after every tape when you make tests.
Since you said that you are refurbishing your machine, I assume that you have lubricated the capstan motor shaft with PDP65. If not, that may possibly be a cause to
The correct replacement bearing is 626-ZZ, from a good make (SKF is the standard choice - no affiliation, just experience). It is waterproof so it cannot be lubed, you have to change it, but since it is cheap (3.90 € a piece) and easy to find this is not a problem. As for the shims, there is normally a 0.15 mm shim on each side of the bearing, plus a shim or two (0.3 mm total thickness if memory serves) underneath the lower cup. Adding shims gives more leeway for badly slit tapes, but it increases phase fluctuation very noticeably so I only do it when needed and then remove the extra shims for normal use.
This is rather extreme, especially considering the precision required to keep the flutter low with this motor. I only pull it apart as a last resort. The recommended lubrication procedure is to just add a few drops of Isoflex PDP 65 in the space between the capstan shaft and the plastic cap, in order to refill the felt ring underneath which is the oil reservoir of the motor, and then let the latter run for a few hours, preferrably with the deck in horizontal position. It is enough to keep the capstan motor lubricated. Whatever you do, make sure to use only PDP 65: mixing oils in sintered bearings always causes problems. PDP 65 is not cheap, but it is plentiful on a well-known auction site and 3 ml is a lifetime supply for a single Revox, so penny-pinching is a bad policy here. It is only if the motor does not spin freely after this procedure that you should consider opening it, but then, if it is still not free after cleaning and lubrication, there is not much you can do apart from changing the whole motor: I don't think spare parts are still available.