Starter problem????

I'll describe the problem first then what I've done so far. Any help would be great.
Problem: When cold, Glow plug light goes out, crank motor and it is a short crank. It sounds like while the key is in the start position that the starter disengages and spools down. If I plug it in and the motor is warm, no problem.
What I ve done: Replaced starter relay ($12.95) and removed and cleaned all electrical connections to batteries, starter and relay.

I'm leaning toward the solenoid on the starter is going. I figure with the additional resistance mechanically when the motor is cold, and the solenoid being bad, it craps out mid crank. Just a thoery at this point. I'd like to make sure before dropping the coin on a new starter.
Thanks in advance for the help. /ubbthreads/images/graemlins/shrug.gif