I have a 1994 325i conv. the temp gauge cycles between normal operating temp and overheating as i'm driving.a few minutes in each range.could it be the radiator.blocked passages? is this common to older bmw's

have you checked your thermostat to make sure its operating at correct temps? they are usually less than $10 and easy to install.

Overheating Due to corrosion block in Water Pump on Erratic engine coolant temperature or intermittent overheating can be caused by corrosion inside the water pump. ...Either condition reduces coolant circulation resulting in engine overheating.

First question is what constitutes "overheating".
Since the car has been parked for years, the thermostat could be sticking closed.
The cooling system could have air pockets and need to be bled.
The water pump may be corroded inside and not pumping enough volume.
If the gauge goes above half way, the fans should start up. But half way should be more or less normal, not hot yet.
I would look at flow problems first, beginning with removing the thermostat and driving it without one for a few days. Also check and see if there is a process to bleed the cooling system of air.

Check coolant level, and possibly replace thermostat. When a thermostat gets stuck in the closed position, coolant does not circulate thru motor. Gauge stays midrange when driving, because motor is being cooled by air and or the fan. If car seems to operate completely normal, then the gauge is faulty.

check your coolant level. half way on the guage is normal

195 degrees is the typical high end on the thermostat (when the thermostat opens), so the temp gauge should read a max of 195 degrees, give or take a few degrees. Some people replace their thermostat with a lower temp one, such as a 180 degree one. Then the gauge will read lower (approx 180 degree max). So the range is typically from cold at start to just under 200 degrees.

is the temp gauge sender new
or the engine cooling fans spinning fast enough
air in the heating pipes ? bleeding

You need to check the water pump and coolant level. If coolant level is low you might have the air pocket in system. Overheating of the car can cause the damadge of head gaskette and engine block itself.
Good Luck

The Temp gauge should read in the middle of the gauge at operating temperature. This is normal. The radiator fan should come on after idling (not driving) for 20 minutes. The fan should also come on when you turn on the AC. Are you experiencing overheat problems?
Please reply if you have overheating issues.

It may actually be a coolant temp sensor problem - they do have issues sometimes. I'd replace that before anything else - they're generally inexpensive (in the $20 or less range). If, when you ask for a new one, they ask you which one you want, you need the one for the dash gauge. Some cars have one for the gauge and one for the car's ECU. Your problem is with the one that drives the gauge, so get that one.