You've got to find out if it's a fuel or ignition problem.
Next time it dies, watch the tachometer (hopefully it's got one). If the tach needle drops right away or flickers just before it dies, it's probably spark. If the needle slowly falls as the power drops it's most likely fuel.
A fairly common fuel problem with symptoms like yours is the fuel pre-pump in the fuel tank. I had that replaced on my '84 240 wagon a while back. Made quite a difference. But first do the check suggested by the previous poster.
hey mine does that too, or it did till it wouldnt rev past about 2500 (no tach). now it wont start at all. i changed the in tank pump now i get 90-110 psi past the regulator. i changed the plugs, wires and the coil. i got one of the awsome in line spark checker and it gets A LOT of spark now. if i spray starting fluid it will run so i know its fuel. im going to crack open the fuel line at the injector to see if it gets gas if it does then its the injectors if not its the fuel distributor. ill keep yall posted.
i had this problem a while back, it would start up great, id be driving and after a few minutes it sometimes would just shut off, i checked the fuses, replaced a few bad ones and fixed the fuse clamps and it hasn't done it since.