我的keras神经网络模型的准确性和损失不稳定

我已经借助keras为二进制分类问题构建了一个NN模型，下面是代码：

# create a new model
nn_model = models.Sequential()

# add input and dense layer
nn_model.add(layers.Dense(128, activation='relu', input_shape=(22,))) # 128 is the number of the hidden units and 22 is the number of features
nn_model.add(layers.Dense(16, activation='relu'))
nn_model.add(layers.Dense(16, activation='relu'))

# add a final layer
nn_model.add(layers.Dense(1, activation='sigmoid'))

# I have 3000 rows split from the training set to monitor the accuracy and loss
# compile and train the model
nn_model.compile(optimizer='rmsprop',
              loss='binary_crossentropy',
              metrics=['acc'])

history = nn_model.fit(partial_x_train,
                    partial_y_train,
                    epochs=20,
                    batch_size=512, # The batch size defines the number of samples that will be propagated through the network.
                    validation_data=(x_val, y_val))

这是训练日志：

Train on 42663 samples, validate on 3000 samples
Epoch 1/20
42663/42663 [==============================] - 0s 9us/step - loss: 0.2626 - acc: 0.8960 - val_loss: 0.2913 - val_acc: 0.8767
Epoch 2/20
42663/42663 [==============================] - 0s 5us/step - loss: 0.2569 - acc: 0.8976 - val_loss: 0.2625 - val_acc: 0.9007
Epoch 3/20
42663/42663 [==============================] - 0s 5us/step - loss: 0.2560 - acc: 0.8958 - val_loss: 0.2546 - val_acc: 0.8900
Epoch 4/20
42663/42663 [==============================] - 0s 4us/step - loss: 0.2538 - acc: 0.8970 - val_loss: 0.2451 - val_acc: 0.9043
Epoch 5/20
42663/42663 [==============================] - 0s 5us/step - loss: 0.2526 - acc: 0.8987 - val_loss: 0.2441 - val_acc: 0.9023
Epoch 6/20
42663/42663 [==============================] - 0s 4us/step - loss: 0.2507 - acc: 0.8997 - val_loss: 0.2825 - val_acc: 0.8820
Epoch 7/20
42663/42663 [==============================] - 0s 4us/step - loss: 0.2504 - acc: 0.8993 - val_loss: 0.2837 - val_acc: 0.8847
Epoch 8/20
42663/42663 [==============================] - 0s 4us/step - loss: 0.2507 - acc: 0.8988 - val_loss: 0.2631 - val_acc: 0.8873
Epoch 9/20
42663/42663 [==============================] - 0s 4us/step - loss: 0.2471 - acc: 0.9012 - val_loss: 0.2788 - val_acc: 0.8823
Epoch 10/20
42663/42663 [==============================] - 0s 4us/step - loss: 0.2489 - acc: 0.8997 - val_loss: 0.2414 - val_acc: 0.9010
Epoch 11/20
42663/42663 [==============================] - 0s 5us/step - loss: 0.2471 - acc: 0.9017 - val_loss: 0.2741 - val_acc: 0.8880
Epoch 12/20
42663/42663 [==============================] - 0s 4us/step - loss: 0.2458 - acc: 0.9016 - val_loss: 0.2523 - val_acc: 0.8973
Epoch 13/20
42663/42663 [==============================] - 0s 4us/step - loss: 0.2433 - acc: 0.9022 - val_loss: 0.2571 - val_acc: 0.8940
Epoch 14/20
42663/42663 [==============================] - 0s 5us/step - loss: 0.2457 - acc: 0.9012 - val_loss: 0.2567 - val_acc: 0.8973
Epoch 15/20
42663/42663 [==============================] - 0s 5us/step - loss: 0.2421 - acc: 0.9020 - val_loss: 0.2411 - val_acc: 0.8957
Epoch 16/20
42663/42663 [==============================] - 0s 5us/step - loss: 0.2434 - acc: 0.9007 - val_loss: 0.2431 - val_acc: 0.9000
Epoch 17/20
42663/42663 [==============================] - 0s 5us/step - loss: 0.2431 - acc: 0.9021 - val_loss: 0.2398 - val_acc: 0.9000
Epoch 18/20
42663/42663 [==============================] - 0s 5us/step - loss: 0.2435 - acc: 0.9018 - val_loss: 0.2919 - val_acc: 0.8787
Epoch 19/20
42663/42663 [==============================] - 0s 5us/step - loss: 0.2409 - acc: 0.9029 - val_loss: 0.2478 - val_acc: 0.8943
Epoch 20/20
42663/42663 [==============================] - 0s 5us/step - loss: 0.2426 - acc: 0.9020 - val_loss: 0.2380 - val_acc: 0.9007

我绘制了训练和验证集的准确性和损失：

如我们所见，结果不是很稳定，我选择了两个历元来重新训练所有训练集，这是新的日志：

Epoch 1/2
45663/45663 [==============================] - 0s 7us/step - loss: 0.5759 - accuracy: 0.7004
Epoch 2/2
45663/45663 [==============================] - 0s 5us/step - loss: 0.5155 - accuracy: 0.7341

我的问题是，为什么精度如此不稳定，而经过重新训练的模型只有73％的精度，如何改善模型？谢谢。