In early stages of sorting out the 1989 190D my friend just bought. When I glow, the light comes on and then I start the car when it goes out. Starts perfectly with no rough idling whatsoever...so apparently all the plugs work. But then, after a couple of minutes, the glow light comes on. What could be the cause of this symptom?

__________________
14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife