You're in too deep now to give up on such a great car.
J
There are two ways to look at this:
1. If this is trying to say "you've already replaced or repaired a significant number of items such that there aren't that many more things that can go wrong", then yes, maybe it is worth keeping it. If you estimate that there really isn't much more that will go wrong, then, yes, invest the remaining $$ and get it fixed and keep it.
2. There is something I learned from the Project Management Institute (a wee bit of self-promotion here: I am PMP certified): when determining whether to continue a project, "sunk cost" is not a determinant. In other words, the forward-looking view should only be considered. It doesn't matter if a project has $1m invested; if the remaining cost to finish does not justify the benefits, then kill the project. Hence the phrase "don't throw good money after bad."
So, on #2, if the anticipated remaining work that might need to be done (and I know it's not easy to quantify) ends up outweighing the tangible value (what the car would be worth all buttoned up) + your quantification of the intangible benefits (how much is joy of ownership worth to you), sell it and move on, regardless of how much $$ has already been spent.
Of course, this is all total BS if you really want to decide this emotionally. But if you want to view it objectively, do as others have said, get a SOLID evaluation of the car's condition, what it would cost to get it where you want it, then decide, FORGETTING ABOUT WHAT IT'S ALREADY COST YOU.
Hope this makes sense. We feel ya, I HATE when my cars have problems. (I usually get p*ssed at them and sell them, even when the costs to repair were nominal and I could have enjoyed them many more years. Once I get past that point of no return where I am mad at the car, it's all over and I move on. I think you have to think the opposite way, where you want to keep the car but should sell it if the numbers don't add up.)
Best of luck either way, and let us know what you decide.