I'm a newbie with a question: my girlfriend recently purchased a '75 3.0Si that idles roughly and has a strange oil/ coolant mixture in the radiator reservoir. A mechanic tried flushing out the system but the problem returned immediately. He says it could be a leaky head gakset or cracked cylinder head. I haven't noticed any smoke coming from the tailpipe at starting. Before I take it elsewhere and spend money unnecessarily, any diagnoses?