My starter is acting funny - need an expert

[orlowted]

I guess there is a chance that the starter is bad, but the puzzling part is that the first one worked fine before the clutch swap. No shims - bolted directly to the bellhousing. I did replace the battery at the same time - I wonder if there is not enough amps - although I did get the same battery as before. When it does catch the engine - it cranks it over for about 2-3 seconds and then just starts spinning. At other times when I turn the key - it will not engage at all and just spin. It is random. I am going to try bypassing the solenoid and putting a direct connection to the starter and see if that makes any different. I am also going to try it with my truck battery instead of the one I bought. I know it is a longshot, but I am running out of ideas. I will let you know how it goes. Thanks for all the advise - this is a tricky one (arent they all with a 41 year old mustang)