Was pulling a nearly empty 28% sidedress rig back to the coop tonight (15th speed, road gear) when all of a sudden the "Trans Pres" light came on and the tractor started to coast.  I clutched, came to a stop, put it in park, then tried to put it in gear again.  Nothing.  Turned off the tractor engine, started up again, and was able to put it in gear and drive off.  Hmm. 

On the return trip (not pulling anything) it did this to me twice again.  Loose forward motion, Trans Pres light comes on, STOP light comes on with warning.  I can turn the engine off, back on again, and can drive (but for how long?).

Any ideas on what may be going on?  1986 JD 4450 with 15 spd powershift.  Thanks!