Do you have a schematic that matches your unit? It is always nice to post that along with your question so that we can follow along easily, and also tell the component number of the failing parts. The schematic I found says Series C, which implies there is an A and a B as well.
There are a lot of power transistors in CS800 in parallel banks, if you replace a couple at a time (even same manufacturer and model) you could have considerable mismatch on the transistor parameters, which may lead to others failing down the road. Also, are you getting transistors from major dealer or from something like Ebay? Lots of fake power transistors on ebay.