I think that the free dealer diagnostics are a very useful tool for the DIY Volvo owner-- I have used the dealer diagnostics myself-- my 2000 V70 had harsh shifts that were fixed by a free software download and some adaptations. VIDA-DICE is great for diagnostics, but unless you have a subscription, you won't have access to software downloads that you may need. That's why I suggested the dealer or an independent. This is particularly relevant in the case of a recurring check engine light with no generic OBDII codes.

The electrical portion of the ignition switch is a potential candidate for some, but in my mind not all of the symptoms the original poster reported. It does not explain the check engine light without codes, or the symptom of the car going into limp mode-- this can be an early sign of an electronic throttle module failure (even without a lit ETS light). I would also measure for voltage drops in the cables, particularly the B+ cable from the positive battery cable to the fuse box.