I've not read your other posts, but my guess would be either a worn or blocked oil pump, possibly because of the engine flush, or a stuck oil presure relief valve it these engines have one (I guess they should).
If your garage are saying they think its the big end/main bearings, ask them to take a couple of the caps off so you can have a look at the bearings for yourself. If there is any scratching or if the white metal has worn away and they have orangey patches, or if your crank is scored then you need to get the crank out and checked and reground if needed, and new bearings to suit.