I have a basic feedback form on my page. This is the surface controller...

using Umbraco.Web.Mvc;
using System.Web.Mvc;
using InstallUmbraco.Models;
using System.Net.Mail;

namespace InstallUmbraco.Controllers
{
    /// <summary>
    /// Surface controller for contact form model
    /// </summary>
    public class ContactSurfaceController : SurfaceController
    {
        public ActionResult Renderform()
        {
            return PartialView("~/Views/Partials/_Contact.cshtml");
        }
        [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult SubmitForm(ContactModel model)
        {
            if(ModelState.IsValid)
            {
                SendEmail(model);
                return PartialView("~/Views/Partials/_Contact_SUCCESS.cshtml");
                }
                return PartialView("~/Views/Partials/_Contact_FAIL.cshtml");
            }

            private void SendEmail(ContactModel model)
            {
                string htmlMsg = "<table cellpadding=\"2\" cellspacing=\"2\" border=\"0\">";
                htmlMsg = htmlMsg + "<tr><td>Name:</td><td><strong>" + model.YourName + "</strong></td></tr>";
                htmlMsg = htmlMsg + "<tr><td>Email:</td><td><strong>" + model.YourEmail + "</strong></td></tr>";
                htmlMsg = htmlMsg + "<tr><td>Message:</td><td><strong>" + model.YourMessage + "</strong></td></tr>";
                htmlMsg = htmlMsg + "</table>";

                string subject = string.Format("Enquiry from {0} - {1}", model.YourName, model.YourEmail);

                MailMessage message = new MailMessage("website@mywebsite.com", "sales@mywebsite.com", subject, htmlMsg);
                message.IsBodyHtml = true;
                SmtpClient client = new SmtpClient();
                client.Send(message);
            }
        }
    }

The Partial view looks like this...

@inherits UmbracoViewPage<InstallUmbraco.Models.ContactModel>
<div id="formOuter">
    @using (Ajax.BeginForm("SubmitForm", "ContactSurface", null, new AjaxOptions
    {
        HttpMethod = "POST",
        InsertionMode = InsertionMode.Replace,
        UpdateTargetId = "formOuter"
    }))
    {
        @Html.AntiForgeryToken()
        <div class="grid-x" id="js-contact-form">
            <div class="cell small-12 redText" id="js-contact-form-errors">
                @Html.ValidationSummary()
            </div>
            <div class="cell small-12">
                <label>
                    Name:
                    @Html.TextBoxFor(m => m.YourName, new { placeholder = "Your Name" })
                </label>
            </div>
            <div class="cell small-12">
                <label>
                    Email:
                    @Html.TextBoxFor(m => m.YourEmail, new { placeholder = "your@email.here", type = "email" })
                </label>
            </div>
            <div class="cell small-12">
                <label>
                    Message:
                    @Html.TextAreaFor(m => m.YourMessage, new { placeholder = "Your message...", rows = "5" })
                </label>
            </div>
            <div class="cell small-12">
                <button class="button use-ajax">Submit</button>
            </div>
        </div>
    }
</div>

It seemed to work well, but after reboots, etc. I get the YSOD with the following info...

Unable to resolve type: InstallUmbraco.Controllers.ContactSurfaceController, service name:

Can anyone help or explain why this is happening periodically?

Thanks in advance!