Hi I have a 91 eb that I have just swapped the engine and gearbox from a 92 eb that was running fine. Once I got it all connected and serviced went to go for a test drive and got about 500m down the road and 4th gear dropped out along with my speedo. I’ve been told it’s in limp mode. I swapped the battery out and it all came good then the next day back to limp mode. Anyone kno how to fix this?