Not sure what the OVP relay does to the engine if you car has HFM engine management system. It could certainly cause some of these symptoms in a KE-Jetronic car, but my HFM car runs fine with the OVP relay removed. OVP failure will always bring on the ABS warning light. If that light's not on, then OVP is OK.
I suggest you check for vacuum leaks before spending any money - this would tie in with the poor shifting.
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)