BMW z3 - going thu radiators like tissues

[Corwen1]

My Z3 1.9i 8v (2001) - which I've had for just over a year - keeps busting its radiators. Counting the one that was on it when I got it last year, it's just had its fourth one fitted. The garage I got it from wasn't a BMW garage and gave me a guarantee (which is now up). Within a month of getting it last year, the rad went on motorway and left me stranded miles from home. New one was fitted and cap, I think. I collected the car and it did same again within a few miles. AA got me home and back to garage I got it from. They then had it in for ages and fitted a new rad etc. (that was 3). It's been fine all this year till a couple of weeks ago when the coolant was all down my drive when I went out to the car in the morning. It hadn't had any hard use the day before. Mechanic friend tells me, all this points to a bigger problem - like head gasket etc (big job?) but garage say they've checked all that. Fourth rad now on but I'm not happy - telling garage I either want them to buy it back from me or an independent engineer's report. Can't feel confident in the car any longer. Anyone have any ideas what might be causing this recurrent problem please? Many thanks in anticipation!!