problem starting 1988 xj6

We have recently purchased a 1988 xj6 and took it about 25 miles over the weekend. It was running fine then we stopped got out for about an hour and when we returned it would try to turn over but would not. We did have it jumped ( we do know now that we shouldn't have) and by pressing the accelerator to the floor it started. We drove it home, which is 25 miles and all was fine. We checked it yesterday and it started right up I backed up about a foot and the accelerator didn't seem to work it smoked and stopped. It looks like I will have to have it towed quite a ways to have it looked at. Does anyone have any ideas of what we are looking at? Any help will be greatly appreciated. Thanks in advance