Engine and transmission problem

My problems started with the engine going full throttle at start up.. After I started it again it would surge between 2000 and 3000 rpm. I talked to a chev. Mechanic and he told me to hit the ecm. It dropped to an idle then ran like nothing happened. I replaced the ecm with a reman and followed the relearn procedure resulting in no start. The NAPA tech told me to reinstall my old ecm and it ran fine. ( Bad ecm ) I then drove the truck to a reputable mech. On the way it sped up to 85 mph while riding the brakes ! After a rough section of road it settlrd down and drove normally. The mech put the truck on his machines and could not get any codes except a one time low oil pressure. They could not get the truck to misbehave ! I told them to drive it on a rough road. That is when it did the same thing. They then called me to tell me that it was so bad the had to tow it in. Oh and by the way. Now the transmission is stuck in drive ! No matter where you place the shifter it is in drive ! Their diagnosis was a bad injector pump and replace the oil pressure sending unit and pigtail since it is oil soaked and been spliced more than once. No guarentee that wouild fix it and then deal with the transmission ! Could all of this be related since both the engine and trans are controlled by the ecm / Bad ground? HELP I don't want to spend another dime on this truck and am thinking of junking it !