At times, 95 E420 won't start

[FD5]

My mom's 95 E420 won't start at times, she was locking it with an aftermarket remote. When she would lock then unlock with the key it started fine. I told her not to use the remote anymore and just use the key. It has been a month or so since she has had any problems until today. She used the key to lock the car and came out about 20 minutes later and unlocked it with the key but the car won't start, after about 10 minutes it started.