I've been in the same spot before. I know how much it sucks.
How far away is the nearest complex aircraft that you can rent? Also, what type of problem was your flight school having with the aircraft? Does your flight school have a history of an unreliable fleet?
If the problem is an ongoing problem that is likely to come and go, I'd recommend trying to do an intense couple days of training in another school's aircraft and get the rating finished off. That will be a lot cheaper and easier than continuing to start/stop/plug along at your current school.
However, if the plane is going to get repaired and stay up and running for a while, I'd stick it out where you're at. If your school has treated you pretty well up to this point and this is only one occasion of poor service, you're better off staying with them.
Also, a point to consider, is how much would you tick them off by going elsewhere in order to finish? Would it affect your employment with them as a CFI once you get your CFI ticket?
When I was in a similar situation it was during my commercial training. Over the course of my instrument training I'd had numerous irritations because of their maintenance program. Then, during my commercial training I ran into even bigger problems. The Arrow I was using had recurring landing gear problems. It would go into the shop for repairs, be down for several weeks, return to service, then go down again a few days later because they never found the true source of the problem. While all this was happening I had been doing ground work for my CFI.
After three months of hassle I was fed up enough to leave. I spent a week and a half in an accelerated program and finished off both my commercial and CFI. The first school wasn't especially happy with me, but they were shorthanded enough on instructors that they hired me anyway. I did my CFII with them later and it's all good now. I don't regret any of my decisions and I would do it all over again the same way if I needed to.