All as above Oldsinner hit it on the head , simple maintenance first . Or get a diagnostic auto guy to find the problem .You can spend lots of $ and be wrong . I had a problem last year, and rather than spend cash on it , and not knowing just what it was . He came and found it right away .The coil was shot . I could have put all new parts on it till i found the problem .And would cost quite a lot to fix it .And the coil would have been last on my list.